Chapter 6 5. Optics

introduction This article attempts to explain how light enters the eye and is redirected when it encounters various objects -- the phenomenon known as refraction.And observe and measure the refraction through experiments.On this basis, the general nature of sensation, especially the imaging process of vision is discussed. first lecture How we act in life depends entirely on feeling.Sight is the chief and most exhaustive of all the senses, and the many inventions which augment the powers of sight are undoubtedly the most valuable of all.And we would hardly find a more useful invention than that which enhances vision, the wonderful telescope.Although the telescope has only been in use for a short period of time, it has already discovered more novae and other celestial objects than we have ever seen combined.We can see farther and farther than our ancestors usually imagined.The invention of the telescope seems to have opened a way for us to acquire a greater and more complete knowledge of nature than our ancestors did... Some complex inventions have not yet been perfected, and I can still have enough doubts to discuss .Since the manufacture of the inventions in question depends on the skill of artisans, who generally have no formal education, I will try to use other aspects of scientific knowledge to make myself known to everyone, and try not to overlook something.I shall, therefore, begin by explaining light and rays, then briefly describe the structure of the eye, and discuss in detail the production of vision; make it happen.

The only reason I speak of light here is to explain how it enters the eye and how it is diverted when it encounters various objects.I don't want to explain what the nature of light is.I think the concept of light is easier to understand if you use two or three metaphors, which are most suitable for explaining the properties of light.We can know some of its properties through experiments, and then use them to infer other properties that are not easy to observe.Here I am following the example of astronomy, whose assumptions are almost always false or inconclusive, but which yields many very true and certain results, all related to a large number of observations.  Everyone must have such an experience. When walking on an uneven road at night without lights, it is necessary to use a small branch to find the way.You may have noticed that with this twig, you can figure out whether there is a tree, stone, sand, water, grass, dirt, or something else in front of you.Indeed, this feeling can be a bit confusing and incomprehensible to those without long-term experience.But those who are born blind, they use this sense throughout their life.Some claim they can see with their hands or that a twig in their hand gives them a sixth sense instead of sight.When you think about these situations, it feels so perfect and precise.

For comparison, we can consider the light in a so-called "luminous" object.This kind of light is nothing else, it is just some kind of movement or extremely rapid activity, which is transmitted to our eyes through the medium of air or other transparent objects, just as the blind man can transmit the obstacles encountered by movement or objects to his hands through crutches .An obvious example is the light of the sun, which emits light that reaches us instantly.We know that the act of moving one end of a crutch must be transmitted to the other end in an instant, and the light must travel from the sky to the earth in the same instant, although the distance between them is much greater than the length of the crutch.Another obvious example is that through this act of movement, we can see various colors.Everyone may believe that the colors in what we call "colored" objects are just the various variations in which the object receives light and sends it to our eyes.We all know that the difference between trees, stones, water, and similar objects that a blind man finds with a stick, and the difference between red, yellow, green, and other colors that we see, and the difference between objects that a blind person feels, It's just a difference in the way you move the crutches or stop them from moving, not for anything else.It is therefore justifiable to conclude that it should not be supposed that there is any substance which passes from the body to our eyes to enable us to see colors and lights; still less should it be supposed that there is anything in the body which resembles the thoughts or sensations we have.In exactly the same way, when a blind man feels a body with a stick, nothing flows from it and is transmitted through the stick to his hand. The concept of an object has no similarities either.In this way the human mind arises from all those little images that are fleeting in the air.Philosophers use the imagination, which they call "conscious formation" a "form" or "image" similar to what the scholastics refer to as being transmitted to the mind by objects.

You will find that this approach easily resolves the current controversy among philosophers concerning the origin of vision.The blind person senses the surrounding objects, not only through the movement of the object on the behavior of the crutch, but also through the behavior of the blind person's hand when the object does not move but hinders the crutch.In the same way, we must admit that objects are perceived by our sight not only by the action of the object directly transmitted to the eye, but also by the action of the eye which fixes on it.But since the latter behavior is only a kind of light, we noticed that animals like cats have eyes that can see in the dark, whereas ordinary people can only see things through the behavior of the target.Experiments have shown that objects must be luminous, or illuminated, to be seen, not that in order to see them, our eyes must be luminous or illuminated.The blind man's cane is markedly different from air and other transparent objects, so to make things clearer we must use another metaphor.

Take a look at the large wine barrels during the harvest season, which are filled with half-squeezed grapes. We punched two small holes at the bottom of the barrels, and the unfermented wine can flow out of them. The picture of the barrels is omitted here.Now observe, there is no vacuum in nature, (almost all philosophers admit) there are many small pores in the objects we see around us (experiments can well prove it), these small pores must be filled with some very small And very fluid substances, which are constantly being transported to us from the celestial bodies.If you compare these tiny substances with the wine in the barrel, if you compare the less liquid, less clean air and other transparent bodies with the bunch of grapes in the barrel, it will be easy to understand what follows.The moment the small hole at the bottom of the barrel is opened, at the same time some wine tends to fall vertically out of one small hole, and also tends to flow out of the other small hole; Hole.These acts do not hinder each other, nor are they hindered by bunches of grapes in barrels.The bunches of grapes in the barrel are supported by each other, and there is no tendency to drop and fall out of the small holes like wine.Even so, the wine will undergo the above changes.At the same time, the wine can even be moved in many other ways by squeezing the bunches.In the same way, when the sun is facing us, the tiny particles of matter on its edge tend to shoot straight to our eyes for a brief moment when they are let go.These particles do not interfere with each other, and even the unclean parts of the transparent objects they pass through will not hinder them, and no matter how these transparent objects move, such as air is almost always blown by the wind, glass This happens both with the crystal standing still.Note here that it is necessary to distinguish between movement and behavior or tendency of movement, and it is not difficult to imagine a certain portion of wine tending to flow to one orifice as well as another.They cannot, in fact, flow to both holes at the same time, they are perfectly inclined to flow to this or that hole in a straight line, although the influence of the bunch of grapes prevents them from being quite in a straight line.In the same way, considering that the light on a luminous object is mostly regarded as its behavior rather than its motion, we should think of light as just some lines to which its behavior tends to point.So there is an eternal line that goes from all points of a luminous body to all points of an object illuminated by it.As you can imagine, the "behavior" generated from all points on the surface of the wine tends to flow along a straight line to a small hole; the "behavior" generated at the same point will also tend to flow along other lines to other holes. A small hole, these lines of different trends will not hinder each other.

Moreover, we can imagine these lines to be straight as they pass through a homogeneous and completely homogeneous transparent body.But when they encounter other objects, they are prone to diverting or weakening, just like a moving ball or a stone thrown into the air changes direction when it hits an object.We can easily believe that the behavior or tendency of motion (I have already said, mainly talking about light) must in this respect obey the laws of motion itself.To give a complete explanation of the third analogy, consider a ball flying through the air, which may encounter a soft, hard, or fluid body, which, if soft, can completely block the ball and Stop it if it hits a linen sheet, sand, or dirt; if it hits a hard object, the ball will be bounced in the other direction instead of stopping it.There are many different modes of action here, since the surface of these objects may be very smooth or rough; or in the smooth case, it may be flat or curved; The matteness may simply consist of many smooth surfaces of various kinds, or it may be that the object itself has many corners, or that some parts are stiffer than others, or that some parts are in motion (there are several in a thousand different ways).

We must notice that besides the simple and general motion of the ball from one place to another, there is another way, that is, to rotate itself around the center of the ball. The speed of the latter may be related to the former in various ways, so , many balls thrown from the same direction bounce back in unison in the same way when they hit a body with a perfectly smooth surface.If the surface of the object is perfectly flat, the balls will bounce back the same distance as before when they hit it; but if the surface of the object is curved inwards or outwards, the balls will collide or fly away from each other In the same way, more or less, depending on the curvature of the surface... It must be known that there are many objects which act in the same way to stop the light and absorb all its power.Such objects are called "black" objects, having no color other than "black" like shadows); certain other objects, which reflect light, some of which reflect in the same order as they were received (i.e. Objects with highly smooth surfaces, whether flat or curved, can act as mirrors), others of which reflect light in a variety of different directions, in a state of complete chaos.In the latter, some objects do not bring about any change in the behavior of the light when it reflects it (i.e. what we call "white" objects), while other objects bring about some other change.Similar to what we see when we stare at a moving ball (i.e. red, yellow, blue, or some other such colored object).I believe it is possible to determine the nature of a color and reveal its mysteries experimentally, but this is beyond the scope of my subject see "Description of the Human Body". .All I'm trying to do here is point out that when light hits an object, when the object has color and is matte, even if the light comes from a single direction, it is always reflected in all directions... Finally, take into account that light can also be skewed , like the motion of the ball just described, when a ray of light strikes obliquely the surface of a transparent object and passes through it, it can easily be skewed, skewed, or increase, or decrease.This way of skewing is called refraction.

Lesson 2 Refraction Then we must know how to measure exactly this amount of refraction.The metaphor I have just used is enough to make this matter easy to understand.In my opinion, first discuss reflection to make it easier for us to understand refraction (picture omitted).Let us assume that the ball is shot from A to B and intersects the ground CBE at point B. The ground CBE prevents the ball from moving forward and turns it around. Let us see which direction the ball will move.In order to avoid unnecessary trouble, we assume that the ground is perfectly flat and hard, and that the ball is always moving at a constant speed, whether it is going down or bouncing, regardless of the fact that the ball is no longer in contact with the racket. force, without taking into account any weight, size or shape effects.Because it is useless to discuss such details here, none of these factors are included in the behavior of light, which is what we are looking for at present.We need only note that the force, whatever it may be, which causes the ball to go on, is not the same as the force which determines its motion in one direction and not in another.This problem can be easily understood from the following facts.The motion of the ball depends on a force, which was once imparted by the racket, which allows the ball to fly in any other direction as if it were point B.

Likewise, the tendency of the ball to move toward point B is determined by the position of the racket.Even with another force at play, the racket will still determine the motion of the ball in the same way.It is already known that the ball will definitely turn when it hits the ground, so the factors that determine the movement of the ball to point B must have changed when the force has not changed.These are two different things, and we cannot imagine, as many philosophers do, that the ball must stop at B before turning to F, because once the motion of the ball is interrupted by such a stop, we shall find that there will be no What caused it to start moving again.

In addition, it must be noticed that not only the factors determining the movement in a certain direction, but also the movement itself can be divided into as many parts as we can imagine. It can be easily imagined that, The factors that determine the flight of the ball from A to B are composed of two parts, one makes it fall from the AF line to the CE line, and the other makes it fly from the AC on the left to the FE on the right at the same time. AB flies to B.It is easy to understand that when the ball hits the ground, it can only stop one of these two factors, but has no effect on the other.Since the ground takes up all the space below the CE line, it must be preventing the factor that makes the ball fall from AF to CE.But why would the ground block another factor that moves the ball to the right?We see that the ground is not all against it.Then, in order to reveal exactly in which direction the ball will bounce, we may draw a circle centered at B and passing through point A, and we consider that the time it takes for the ball to return from B to a point on the circle must be equal to the time it takes for the ball to bounce from A to A. It takes the same time to fly to B, and the distance from B to all the points contained in the circle is equal to the distance from B to A.Assuming that the ball is moving at a constant speed, then in order to determine to which point on the circle the ball must return, we draw three straight lines AC, HB, FE, all perpendicular to the line CE, and the distance between AC and HB is the same as the distance between HB and FE. The distance is the same.We assume that in the time it takes the ball to move to the right from A (a point on line AC) to B (a point on HB) the ball can travel from all points on line HB to corresponding points on line Corresponding points on AC are equidistant.The movement of the ball towards the FE side is determined by as many factors as before.It is the case that the ball cannot reach a point on the line FE and another point on the circle AFD at the same time, unless the same point is D or F, the only two points where the circle intersects the line.So from the ground preventing the ball from going to D, it must be inferred that the ball must go to F, and from this it is easy to see how the reflection takes place: that is, at the same slope as the so-called angle of incidence.Similarly, if light falls from point A to point B on the surface of the plane mirror CBE, it will be reflected to F, and the reflection angle FBE is equal to the incident angle ABC at this time.

Now let's look at refraction.First of all, we assume that the ball is shot from A to B. Point B is not the ground but replaced by a very thin layer of linen CBE woven with very fine threads. At the same time lose part of the speed (such as suppose to lose half).In view of this, in order to see what course the ball follows, suppose further that the motion of the ball is quite different from the factors which determine its direction—the factors which determine its motion in one direction and not the other; factors, the magnitude of which can be detected separately.Let us also assume that we can imagine that one of the two factors determining the direction of motion, i.e. the one which the ball tends to move downwards, can be somehow altered by collision with the linen, the other causing the ball to go to the right The factor of motion remains the same throughout.Because in this direction linen offers no resistance at all.We then draw a circle AFD (not shown) with its center at B, and three straight lines AC, HB, FE at right angles to the line CBE, the distance between FE and HB being twice that between HB and AC.We see that the ball must move towards point I.Since the ball loses half its speed in passing through the linen CBE, it takes twice as long to fall from B to a point on the circumference AFD as to go from A to B on the linen.Since the ball loses no factor of its previous movement to the right, it must travel twice as far in the same direction to the right in twice the time it takes to get from line AC to HB.Moreover, the ball must reach a certain point on the circle AFD at the same time as it reaches a certain point on the straight line FE.Only through I, which is the only point under the linen CBE where the circle AFD intersects the straight line FE. Suppose now that the ball, being swung from A to D, does not hit the linen at point B, but hits the water.The water, like the linen, attenuates the ball by exactly half its speed, all else being equal.I then think that the ball, passing B in a straight line, is not towards D but towards I, and it must first be that the water deflects the ball from that point in the same way as the linen.It can be seen that the surface of the water weakens the ball by the same amount as before, and also against the ball in the same direction.Although the space between B and I is filled with water, which has a more or less resistance to the ball than we previously assumed, air does, but we say that it is not for this reason that the deflection of the ball by water A little more or a little less.When the water is divided to allow the ball to pass, it is as easy in one direction as in the other, at least if we assume the following.We always assume that the motion of the ball is not altered by its weight, its size or shape, or other such irrelevant reasons.Here we note that the more inclined the ball is at which it hits the water or the linen, the greater is the deflection caused by their action.So if the ball hits the water or the linen at a right angle (as if the ball is shot from H to B), it will go down the straight line and go to G without any deviation.But if the ball, when struck along (not shown) lines like AB, is so inclined that the line FE (drawn as before) does not intersect the circle AD, the ball cannot go through them at all, It can only bounce back into the air from point B on its surface, as if the ball hit the ground in the same way at that point.It is a sad experience that people sometimes experience when playfully firing cannon into the river only to injure innocent people on the other side of the river. Let’s make another assumption here, assuming that the ball is shot from A to B for the first time, and then it hits the racket CBE at point B and is bounced again, which increases the power of its movement, for example, by one-third, then now the ball The distance traveled in 2 seconds will be the same as the distance traveled in the previous 3 seconds.When the ball passes through the plane CBE, if it encounters an object with this property at point B, it will travel one-third faster than in the air (figure omitted).The above two cases have the same effect, which is obviously very similar to what we have shown. If we draw a circle AD and the lines AC, HB, FE as before, then the point I as the intersection of the straight line FE and the circle AD represents The position where the ball will point after the deviation at point B. Now we can also draw the opposite conclusion to the above. It can be said that the ball flying in a straight line from A to B deviates at point B and moves towards I, which means that the force or ease with which the ball penetrates the object CBEI The degree is related to the force or ease with which it leaves the object ACBE.Just as the distance between AC and HB is related to the distance between HB and FI—that is, as the length of the line CB is related to the length of the line BE.As far as the behavior of light is concerned, in this respect obeying the same laws as the motion of a ball, it can be said that when light passes obliquely from one transparent body into another, it is easier than in the first case or more difficult.Deviations take place in this way: on the faces between these transparent bodies, the inclination of the light is always gentler on the side of the more permeable objects, and this inclination changes exactly in accordance with the degree of transparency of the respective objects. Proportionality is not described in detail here. The law published by Descartes is now called Snell’s law. According to this law, sini=nsin, where i is the angle of incidence, r is the angle of refraction, and n is a specific refraction medium For constants, see letter to Mossenay, June 1632.Careful attention must be paid to the fact that this inclination can be measured by the amount of comparison between straight lines (such as CB or AH, EB or IG, etc.) and not by angles such as ABH or GBI, and less often by Angles like the DBI are measured at what we call the "angle of refraction".Because the ratio or proportion between these angles varies with the degree of inclination of the light rays.On the contrary, the ratio or proportion between the lengths of the lines AH and IG, etc., remains constant in all refractions which occur in the same body.For example (figure omitted) suppose a ray of light passes through the air from A to B, and hits the surface of the lens CBR at point B, and the ray will be deflected to I in the lens; suppose another ray is emitted from K to B, and is deflected to L ; moreover a ray of light from P to R is deflected towards S, in which the same proportionality must exist between the lines KM and LN or PQ and ST as between AH and IG.But the same proportional relationship does not exist between the angles KBM and LBN, or PRQ and SRT, as between ABH and IBG. Here you will see how to measure refraction.In order to measure its size, we need to resort to experimentation.As far as the amount of refraction is concerned, it depends on the particular properties of the object in which it is placed.However, we can easily and adequately measure it, since all refractions are reduced by the same amount in the same way.In fact, to reveal all the refractions that occur at a given interface, it is sufficient to examine the beam.In addition to this, we check the refraction of two or three other beams, and we may avoid all possible errors.Therefore, if we wish to know the amount of refraction occurring at the interface CBR, separate from the air AKP and the lens LIS, we can determine the refraction of the light beam ABI simply by examining the proportional relationship between the lines AH and IG.But if we suspect that the experiment may fail, we must measure the refraction of two or three other beams, such as KBL and PRS, and if we find the same proportional relationship between KM and LN, PQ and ST as between AH and IG, We have every reason to believe in the correctness of the observation. In making these observations, however, one may be puzzled to find that light rays are inclined more strongly in air than in water, and more strongly in water than in glass, when refracted at the interface of water and air, as in the case of The opposite happens in the case of the ball, the ball deflects more in water than in air, and the ball cannot go through the glass at all.For example, (picture omitted) a ball is shot from A to B in the air, and when it hits the water surface CBE at B, it will deviate towards V at B; while for light rays, it will tilt in a completely different direction , from B to I.But if you recall the properties of light which I have summarized, you will not be surprised.At that time I preached that light is nothing but a certain movement or behavior of received tiny matter that fills the pores in other objects.Consider also that just as a ball loses more motion when it hits a soft object than it does when it hits a hard object, it is more difficult for a ball to move on a carpet than on bare ground. The behavior of air is more hindered by air particles than water particles (air particles are soft and not well combined, and tiny substances cannot resist it much) and can give more resistance to water particles.Therefore, it is easier for the tiny particles of a harder, denser transparent body to let light pass through them; Walk. We know the refraction that occurs in water, in glass, and in all transparent objects around us, in this way.It may also be noticed, however, that the rays of light must be refracted on their way out of these bodies in exactly the same way as they were refracted on their way in.Therefore, if a ray of light traveling from A to B on its way from air into the lens is deflected towards I, then the same ray of light traveling from I to B must be deflected towards A when it returns. However, other causes of refraction of other objects (mainly in the sky) can also be found, and that refraction is not reversible.It will also be found that under certain circumstances the beam becomes bent, though only through a transparent body.The motion of the ball is often curved because it is deflected in one direction by gravity, in another direction by the action of us hitting the ball, or for other reasons.Finally, I dare say that the three metaphors just used are apt, and that all the properties we can observe in them are exactly similar to those of light which have been demonstrated in the experiments of light.But I've only explained those that are most relevant to my thesis.I don't want to make you think about other things here, but finally consider the surface of a curved transparent body. The light passing through each point on it deviates in the same way as a plane. When touching these objects, we can also imagine the surface of the curved surface. Each point is the same as each point on the plane.For example, (see Fig. 7 omit) rays AB, AC, AD come from the same beam of flame A, and hit the surface of the crystal ball BCD, the way of their refraction is considered to be the same as when AB falls on the EBF surface, and AC falls on the GHC surface The same is true for the upper AD falling on the IDK face and similar situations.As you can see here, these rays can be focused or diverged, respectively, depending on the curvature of the surface they hit.Well, after clarifying the above problems, it is time to start talking about the structure of the eye. I think that by talking about the structure of the eye, everyone can understand how the light entering the eye is processed to produce vision. The third lecture about the eyes is omitted here.For the English translation of this part and the omitted material below, see Descartes' Discourse on Method, Optics, Geometry and Meteorology. . . . Fourth Lecture An Overview of Sensations I must tell you about the general nature of sensations, of which vision is particularly easy to explain.We know for sure that it is the mind and not the body that is capable of feeling, and that when the mind is absorbed and absorbed in some ecstasy or meditation, we see that the body is touched by various objects without feeling.We know that the mind is properly said to be sentient, not because of its existence as a body with external sensory functions, but because of its existence within the brain and the functioning of what are called "ordinary" senses. See Rules and "Passion". .We have observed that whenever the brain is injured or diseased, it generally interferes with all senses, no matter how healthy the rest of the body may be.Recently we know that the impressions formed by objects on the external organs of the body are transmitted to the mind in the brain through nerves. We have observed various accidents, as long as a certain nerve is damaged, it will destroy the body where the nerve sends out branches. Sensation in one part, without causing diminution of sensation elsewhere. Then, for the description of the functions of nerves and animal spirits in producing sensations and movements, see Essays on Man and Passions. …Philosophers generally assume that in order to have the faculty of sentience the mind must receive the images transmitted to the brain by objects, but we are careful not to assume that what we a scholastic mean transmits to the mind its analogous "form" or "image".In any case the properties of these reflections must be conceived quite differently from the philosophers, who cannot tell us how reflections are formed if the concept of a reflection is defined by the requirement that it resembles the object it represents After the object is formed, how it is received by the external senses and transmitted to the brain by the nerves.The only reason they postulate about images is that seeing a picture readily arouses the brain to imagine the object it describes, so in the same way the brain must be aroused by the little pictures we form in our minds, To imagine and elicit sensations about objects. It should be remembered, however, that the brain can be stimulated by many other things besides images, such as signals, speech, etc., which bear no resemblance to what they refer to.In order to avoid as far as possible any disagreement with accepted views, if we are inclined to insist that objects perceived by the senses do send their images to the brain, we should at least observe that an image is by no means in all respects equal to The represented objects are similar, otherwise there would be no difference between the object and its reflection.It is enough that the image resembles in some respects the object it represents.Indeed, the perfection of a reflection often consists in that it is not, as it should be, like the object it represents.As you can see in engravings, with just a little ink, a little here and there on the paper, they can represent to us forests, towns, crowds, even battlefields and storms; though they remind us of These objects are of various qualities, but in fact have some similarities only in shape.The resemblance is not perfect, because what engravings present to us is formed by showing convex and concave variations on a purely flat surface, and, according to the laws of perspective, the circle is often better represented by an ellipse than by other circles. , a square is better represented by a rhombus than by other squares, and similarly for other shapes.It often happens, therefore, that, in order to make an image more perfect, and better represent an object, an engraving cannot resemble the object it represents.We must now believe that images formed in our brains are produced in the same way.We note that the problem is to know how images give the mind perceptions of the properties of objects like it, which correspond to them, but do not know how to be like them.For example, when a blind man seeks objects with a stick, it is true that the objects convey nothing to him, except that the objects, according to their own nature, cause the stick to move in a different way, and cause the nerves in the hand to move, and the nerves of these nerves then move. Movements are produced in the brain region at which they originate, which triggers sensations in the mind, which have a variety of qualities depending on the movement in the brain. Lecture 5 Images Formed Behind the Eyes You see that in order to have the ability to perceive, the mind does not necessarily have to form some kind of image similar to the objects it perceives, but nevertheless, the objects we see do form very special images behind our eyes. perfect image.Let's use an analogy to explain it simply.Assuming a fully enclosed dark room with only a small hole left, place a glass lens in front of the small hole, hang a piece of white cloth behind the small hole, and keep a certain distance from it, so that the light from the external object can Image formed on white cloth.Now we say that the dark chamber represents the eye; the small hole represents the pupil; the lens should represent the lens fluid, which is the part of the eye that refracts;如果把刚刚死亡的人的眼睛取下来（做不到的话，也可以取牛或其它大动物的眼睛），仔细地切断眼后三层薄膜，会露出这些薄膜包围着的大部分体液，注意不要溅出来。然后用某种足够薄的、能让光线透过的白色物体（如一张纸或蛋膜）盖住暗室的小孔，小孔内放上这只眼睛，那么这个眼睛的前面是各种各样的被太阳照亮的物体，后面是暗室，有你站在那里（除了通过这只眼睛外，没有光线能进入暗室，这只眼睛的所有部分都是完全透明的）。做好这些之后，看看白布，你会惊奇地发现一张通过自然透视形成的外面所有物体的图片--只要确保那一只眼睛维持其自然形状，按照物体距离的远近，无论怎样做都能看到（如果将眼睛多少挤一点，图像便会变得不太清晰这里省略了一个图，文章也有删节。 现在，大家已经在死亡动物的眼睛中看到了图像，考虑一下其原因，如果用眼睛内膜来取代白布的位置，那么就不会怀疑在活人眼睛的内膜上能形成类似的图像——实际上是一种更好的图像，因为活人眼中的体液充满生机、更加透明，其产生映像所必需的形状也更恰当。（可能牛眼中瞳孔的形状不很圆，使图像达不到完美的程度）……物体的映像不仅仅以这种方式在眼睛后面形成，而且还要向后传输至大脑这里省略的内容与笛卡尔在《论人》中所述的相重复。……第六讲视觉当图像传输到大脑内时，它仍然与其所产生的物体有某些相似之处。然而就像上面充分显示的那样，我们不应该认为图像使我们感觉这些物体是通过这种相似的方式--就好像在大脑内还有另外一只眼睛，使我们通过其感觉图像。相反地，我们必须相信是自然赋予的运动形成了图像，并直接作用于身体之主的心灵，使心灵拥有感觉。对此，我将予以详细地介绍。我们用视觉所感觉到的物体的全部性质可被归纳为六个基本性质：光、颜色、位置、距离、大小和形状。首先看一下光和颜色（真正属于视觉的性质）。我们假定心灵具有这样一种性质，使其具有光的感觉的是大脑视觉神经纤维组织区的运动的力量，使其具有颜色的感觉是这些运动的状态。 同样地，通向耳朵的神经的运动使心灵听到声音；舌头上的神经的运动使心灵品尝到味道。总而言之，身体上各种神经的运动，缓和的会使心灵得到痒的感觉，很剧烈的则会使心灵得到痛的感觉，但是在所有这些心灵想象到的概念和引起这些概念的运动之间不一定非得有相似之处。大家都乐于同意这样的说法，当人的眼睛受打击时，好像在眼前看到了无数的火花和闪光，即使闭上眼睛或在很暗的地方也是如此。因此这种感觉可以只归于一击之力。这一击之力就像明亮的光一样使光神经纤维处于运动之中。相同的力量作用于耳朵可使我们听到一种声音，作用于身体其它某些部位会使我们感到痛。这也可被下面的事实所证明：无论何时只要迫使眼睛看一下太阳或其它某些明亮的光，即使随后闭上眼睛，也会在一个短时间内保留其印象，在光消逝的过程中好像仍能看到各种颜色的变化。至少这会说明一个事实：视觉神经纤维被某种力量置于运动状态，通常一旦发生后就不能立刻停止。但是当眼睛闭上后保持在眼睛内神经的运动不足以代表引起它的明亮的光，它只能呈现不太鲜明的颜色，这些颜色在消退过程中不断变化。这正如我已经提出的那样，表明颜色的性质只在于运动的多样性。最后这还可被透明物体中各种颜色的频繁出现所证实。 确实，除了对光的接收方式的多样性外没有别的原因，如云中彩虹的出现就是一个例子。还有一个更清楚的例子，这就是在削磨成许多面的玻璃中能看到类似彩虹的现象。但是我们必须仔细考虑的是，什么决定了所见到的光的数量，即什么决定了每一个视觉神经纤维运动的力量。由于我们看到物体上的光的数量与其物体本身所含光的数量总是不相等的，它与物体的远近和瞳孔的大小成一定比例地变化。这种变化也与物体上各点发来的光线在眼睛后面所占据的面积成比例……除非这些部分在颜色方面有某些不同，否则我们不能区分开所看物体的各个部分。对颜色的辨别依赖于两个因素：一个是物体所有各部分发来的光线聚集于眼睛后面尽可能多的不同的点上，而且别处的光线不能射到这同一些点上；另一个因素是映像在眼睛后面占据的区域内有大量的视觉神经纤维。例如，假定一个物体由一万个能发出光线、并以一万种不同的方式到达眼睛后面某一区域的粒子所组成，其结果可能会同时产生一万种可见的颜色。然而如果我们假定眼睛后面那一区域内只有一千个视觉神经纤维，那么这一万个粒子将只能使心灵区分最多一千种颜色。因此，物体每十个粒子一起作用于一个视觉神经纤维，并各自以一种独有的方式移动它。这些独有的方式组成了物体作用的所有方式，因此每个纤维所占据的区域被认为好像只是一个点，这就是为什么一块农田具有无数不同的颜色，从远处看却全是白色的或蓝色的。 一般地，所有物体从远处看不如在近处更清晰；某个物体在眼睛后面所占据的区域越大，就会看得更清楚。以后我们还必须对这一情况进行特别的关注。谈到位置。即物体上每一部分与我们身体相比较的定位，我们用眼睛比用手能更精确地感觉它。对于它的认识，我们不依赖于任何想象，也不依赖于来自物体的任何行为，而是只依赖于神经发源地大脑中各个微小部分的位置。当布满神经的肢体位置变化时，大脑中微小部分位置的变化却总是很微弱。因此是自然赋予我们心灵这种能力，使我们不仅能够知道物体上各部分所占据的地方，而且还能使我们从这些地方转移注意力，到我们想象到的直线上，这些直线从物体上各部分开始延伸至无穷。以相同的方式，我们已经谈得很多的盲人，将手Ａ转向Ｅ（图略）然后再将手Ｃ转向Ｅ，分布在后一只手上的神经引起了大脑的某些变化，通过这种变化，其心灵不仅会知道Ａ或Ｃ的位置，而且还能知道处于直线ＡＥ或ＣＥ上的所有位置。用这种方式，但无论如何他并不知道或不会想到其双手占据的地方，其心灵却能将注意力转向目标Ｂ和Ｄ，并确定它们占据的地方。 相似地，当我们的眼睛或头转向某一方向时，我们的心灵被大脑中的变化所告知，这些变化是由分布在运动肌肉中的神经所引起的……虽然物体在眼睛内形成的图像是上下倒置的，但仍能够在其真实位置上被看到。对此大家不应该觉得奇怪，这正如盲人通过左手能感觉到（其右边的）物体Ｂ，连同一时间能用右手感觉到（其左边的）物体Ｄ。当盲人用两只手摸到了一个物体时，他并不会将其判断成两个物体。同样，当我们的双眼注视同一个地方时，物体在每个眼里都形成了一个图像，但我们却只是看到了一个物体。与对位置的理解不甚相同，对于距离的理解，不依赖于物体发出的映像。相反，判断距离首先依靠眼睛的形状。正如我们已经说过的，看近处的物体时眼睛的形状与看远处的物体时相比，肯定有些细微的不同。当我们根据物体的远近调整眼睛的形状时，我们以某种自然赋予的方式改变了大脑中的某一部分，以使我们的心灵感觉到距离。通常这一情况发生时，我们并没有仔细考虑。例如当我们用手紧抓住某一物体时，我们会按照物体的大小和形状去调整手，并通过手感觉到它，在这一过程中却不必去思考这些运动。 其次，我们判断距离是通过双眼的相互关系。盲人抓着两根拐杖ＡＥ和ＣＥ（假定他并不知道拐杖的长度）。只知道两手Ａ和Ｃ间的距离和角ＡＣＥ和ＣＡＥ的大小，似乎是通过一种自然的几何学，他能从这一认识中就说出点Ｅ在哪里。相似地，当我们的双眼Ａ和Ｂ转向点Ｘ时（这里省略了一个图），线ＡＢ的长度，两个角ＸＡＢ和ＸＢＡ的大小能使我们知道点Ｘ在哪里。我们可以只用一只眼睛，通过改变其位置而做与此相同的事。此时如果我们使眼向Ｘ方向转动，首先停在Ａ点，随后立刻转向Ｂ点，这就足以使我们产生包括线ＡＣ及两个角ＸＡＢ和ＸＢＡ的想象，使我们能够想象点Ｘ的远近。这是通过一种心理活动所完成的。虽然只是一种简单的想象，但也包含着外科医生的推理。他们在测量不能直接测量的地方时，常使用两个不同的有利点。此外感觉距离，我们还有另一种方法，即通过看到物体形状的清晰或模糊程度并结合光的强弱。如果我们凝视Ｘ（图略），来自物体10和12的光不会正好聚焦于我们眼后的Ｒ和Ｔ，而这些物体在点Ｖ和Ｙ时却会如此。从这里我们可以看到这些物体离我们是比Ｘ更远还是更近些。来自物体１０的光射到我们的眼中会比X附近物体的光要强一些，我们由此判断物体10更近些；来自物体１２的光要比来自Ｙ附近物体的光要弱一些，由此我们判断物体口更远些。最后，我们可能已经从另一方面获得了关于物体的大小、位置、形状和颜色的清晰度的映像，或者只知道来自这一物体光的强度。仅这些便可以使我们在实际上并没有看到物体的情况下想象其距离远近。例如，当我们远远地观察一个物体时，判断其距离远近对于那些经常在近处看到的比那些不了解其大小的要更准确些。如果我们正在看一座被阳光照着的山峰，虽然山峰比其阴影中的一片树林还要远，但却正是树林使我们判断山峰更近些。当我们看海上的两艘船，那艘大的离我们更近些，在大小上它们看起来会相等，但是我们可以利用它们在形状上、颜色上、发射出来的光的不同判断哪一个更远些。 至于我们判断物体的大小和形状的方式，不必再去赘述，因为它们已全部包括在我们判断距离和位置的方式中，即我们判断物体大小是通过我们具有的物体远近的知识和看法，与物体在眼后形成的映像的大小相比较--而非仅仅通过这些映像的大小。很明显，离我们近的物体所形成的映像，是离我们十倍远的物体所形成映像的一百倍大，然而我们并没有看到物体有一百倍大，相反看起来物体几乎一样大，至少其距离没有欺骗我们。同样很明显的是，我们根据对于物体上各部分位置的认识和看法判断物体的形状，而不是根据眼睛中的图像，因为在我们看到圆形和方形时，眼中的图像却经常只是椭圆形和菱形。 为了使大家确定不疑地相信我所解释的视觉形成原理，我将再让大家考虑一下为什么有时视觉会欺骗我们。首先要明白是心灵在看，而不是眼睛在看。心灵不是直接地看，而只是通过大脑看，这就是为什么精神病人和那些睡着的人时常看到或认为他们看到在眼前根本不存在的物体。这只是某种雾气搅乱了他们的大脑，使其中的某些部分产生了正常的视觉，好像这些物体就在眼前一样。其次，因为来自外部的映像是经由神经传输至“共同”感觉的，如果这些神经的位置由于某些非正常原因而发生变化，那么这会使我们看到的物体不在其应该所在的地方……再者，因为我们经常断定，激发我们视觉的映像来自于我们为了感觉而不得不朝着看的地方，所以当它们恰好来自别处时，我们就可能会被轻易地欺骗。 因此，眼睛感染黄疸病的人、或通过黄色玻璃看东西的人、或被关在一个除了通过这种黄色玻璃外别处没有光线能进入的房间里的人，他们会认为看到的所有物体都是黄色的。前面所述的暗室中的人，由于他只盯着小孔上的白色物体，他会把外面物体的颜色看成全是白色的，如果眼睛通过透镜和在镜中看物体，则会断定物体在其本来不在的某点且比原来变大或变小了，或变小的同时颠倒了（当离眼睛有些远时）。这种情况的发生是由于透镜或镜子使来自物体的光线发生偏离，这样眼睛不能清晰地看到物体，只有进行调整才能适当地看到这里省略了一个图，文章也稍微作了删节。那些急于查找问题的人会很容易地理解这一问题，在他们力图去测定凸凹镜中像的位置时，他们会以同样的方式看到古人在反射仪中所犯的错误有多大。我们还注意到，所有我们辨识距离的方法都是很不可靠的。因为当物体在四五英尺以外的地方时，眼睛的形状几乎不会有任何可感觉到的变化，即使物体稍微近一点，眼睛的形状变化也很小，不能从中获得精确的知识。如果看一个很远地方的物体，在连接两眼（或同一眼的两个位置）之间的线和两眼与物体间线所夹的角度也很难有什么变化。即使我们的“普通”感觉本身，似乎也不能收到远于大约一二百英尺外物体的概念，这可被观察月亮和太阳所证实。它们在我们所能看到的最远的物体之列，其直径对我们来说通常好像只有一两英尺--尽管我们有根据知道它们是非常巨大的和非常遥远的。我们能很容易地想象塔和山，哪一个更大些，但是我们不能把月亮和太阳想象得有多么大，所以正确认识不常出现。由于我们不能想象远于一二百英尺以外的物体，结果应该是月亮和太阳的直径在我们看来不会超过一两英尺。 它们的位置也会使我们误入歧途。通常地，它们高挂在天空时，看起来比它们升起或降落时要小些。我们可以很容易地注意到它们的距离远近，因为在它们和我们眼睛之间有各种各样的物体，通过仪器测量，天文学家们清楚地证实它们有时看起来比其它时候要大些，不是因为它们被看到时正在对向一个较大的角度，而是因为它们正在远去。这证明古老光学的公理--认为物体外观的大小与视角成比例--并不总是正确的。我们还想象因为白色的或发光的物体，和通常所有那些具有引起视觉的较大力量的物体，总是看起来比它们拥有较少这种力量时更近些和大些，原因是瞳孔收缩以避免运动的强光，这种运动与引起整个眼睛清晰地看近物的运动--通过这种运动我们判断这一物体的远近--是紧密相联的。如果没有其它运动，在某种程度上同时发生，上述运动几乎不会发生（同理，第三个手指如果不稍微弯曲一点，我们则不能完全地将前两个手指闭紧，好像这两个手指要与其它手指一起闭紧一样）。这些白色的或发光的物体看起来更大些的原因，不只在于我们依赖于距离远近而估计其大小，还在于它们在眼睛后面形成了较大的映像。 必须注意到，眼睛后面遍布着视觉神经纤维的末梢，它们虽然很小，也是有大小的，其每一个中，在某一部分可能受到另一个物体的影响，而在另一部分受到另一个物体的影响。但是在某一个给定时间里，它只能以一种单一方式被移动，因此当其最小部分被某些巨大物体影响，其它部分被不很大的物体影响时，其整体运动是按照最大物体所产生的映像，而不是其它物体。因此假定这些小纤维的末梢是１、２、３、（图略）如来自某颗星的光线在眼睛后面留下一个映像，并在１扩散；也会轻微地到达标着２的六个末梢，（假定没有其它光线到达，只有空中这颗星附近区域射来的微弱的光）。在这一情况下，如果星光足够亮的话，星的映像就会扩散到六个标着２的末梢所在区域，因此大家能够看到，外观相当小的星，由于极远的距离永远不会看起来再更大些，而且即使它们不是很圆，看起来却是圆的--就好像一个方塔，从远处看是圆的，因为在眼中只能显现极小映像的物体是不能看出其形状的。最后，由物体的大小、形状、颜色或光等感觉中的图像来判断其距离，已经表明是很容易出现错觉，在我们来看这些图像经常是很小的与其实物差别很大，而且这些图像中的物体的轮廓很模糊，其颜色也更暗弱。