Descartes’s Dioptrics is more than a mere technical treatise on optics; it is an derivation of the law of refraction in discourse 2, perhaps Descartes’ s single. Dioptrics Ren´e Descartes First Discourse On Light All the conduct of our lives depends on our senses, among which the sense of sight being the most. Dioptrics. Ren´e Descartes First Discourse On Light All the conduct of our lives depends on our senses, among which the sense of sight being the most.

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Skip to main content. Log In Sign Up. And it is difficult to find any of these inventions that has done as much good as the discovery of those marvelous telescopes, which, being in use for only a short time, have already revealed more new stars in the sky, and numerous other objects above the Earth, than we had seen before: But, to the shame of our sciences, this invention, so useful and so admirable, was first found only by experiment and good fortune. And, inasmuch as the execution of the things of which I shall speak will depend upon the descartez of artisans, who ordinarily have not done much studying, I shall attempt to make myself intelligible to everyone, without omitting anything or dioptriics anything known from other sciences.

This is why I dioptrlcs begin with the explanation of light and of its rays; then, having made a brief description of the parts of the eye, I will specifically say how vision operates, and then, having remarked on all the techniques that can make it more perfect, I descarges teach how the field of these techniques may be broadened by the inventions which I will describe.

Now, having no other occasion to speak of light here, except to explain how its rays enter the eye, and how they can be deflected by the various bodies descartees encounter, there is no need for me to attempt to say what its true nature is, and I believe that it will suffice for me to make use of two or three comparisons which aid in conceiving it in the manner which seems to me the most correct to explain all of its properties that experience has made known to us, and then to deduce all the other properties which cannot so easily be noticed.

In this I will be imitating the astronomers, who, although their assumptions be almost all false or uncertain, nonetheless, descarhes they agree with many observations that they have made, never cease to allow the derivation of many very true and well-assured consequences. It is true that this sort of sensation is somewhat confused and obscure for those who are dsscartes used to it, but consider it for those who, descqrtes born blind, have used it all their lives, and you will find that they use it so perfectly and so exactly that it may almost be said that they see with their hands, or that 1 What about Kepler?

And to make a comparison with this, I would have you think that light is nothing other, in bodies that we call luminous, than a certain movement, or a very quick and strong action which moves towards our eyes through the medium of the air and other transparent bodies in the same fashion as the movement or the resistance of bodies encountered by this blind person pass to his hand by the intermediary of the stick.

This example will prevent you from thinking it strange that light can extend its rays in an instant from the sun to us; for you know that diopttics action by which one end of the stick is moved must thus pass in an instant to the ddescartes, and that light must pass in the same way between the Earth duoptrics the heavens, even though there would be more distance.


Nor descattes you find it strange that, by means of it, we can see all sorts of colors; and perhaps you will even believe that these colors are nothing other in the bodies that we call colored than the different ways in which these bodies receive light and send it back to our decartes From which it follows that you will have occasion to judge that there is no need to assume that something material descartex between the objects and our eyes to let us see colors and light, nor that there is anything in these objects which is similar to the ideas or the sensations that we have of them: You can even easily decide the question among them concerning the origin of the action that causes the sense of sight.

For, as our blind person can sense bodies which are around him, not only by the action of these bodies when they move against his stick, but also by the action of his hand when they only resist his motion, thus, we must maintain that the objects of vision can be descartee not only by 3 There is nothing in the objects similar to the descarte that we have of them.

Dioptrkcs, because this action is nothing other than light, it must be said that it is only in the eyes of those who can see in the shadows of the night, such as cats, in which it is found: Picture a vat, at the time of vintage full of half-pressed grapes, and in the bottom of the vat, a hole or two, A and B, have been made through which the soft wine that it contains may flow.

Then imagine that, there being no vacuum in nature, as almost all the diotprics maintain, and there being nonetheless many pores in all the bodies that we see around us, as experience shows us quite clearly, it is necessary descarres these pores be filled with some very subtle and very fluid matter, which extends from the stars to us without interruption.

Thus all the parts of the subtle matter descarets are touched by the side of the sun which faces us, tend in a straight line towards our eyes at the very moment that they are opened, without impeding each other and even without being impeded by the heavier parts of the transparent bodies which are between the two: And note here that a descates must be drscartes between the movement, and the action or inclination to move; for one can very easily believe that the parts of the wine which are for example near C tend towards B, and also towards A, notwithstanding that they cannot actually be moved towards these two sides at the same time, and that they tend exactly in a straight line towards B or A, notwithstanding that they cannot move so precisely towards A in a straight line, due cioptrics the bunches of grapes which are between the two: So that there are an infinite number of such rays which come from all the points of the luminous bodies towards all the points of the bodies that they illuminate, in the same way as you can imagine an infinite number desacrtes straight lines, along which the actions that come from all the points of the surface CDE of the wine tend towards A; and an infinite number of others, along which the actions which come from these same points also tend towards B without the one preventing the other.


Moreover, these rays must always be imagined to be completely straight, when they pass only through one transparent body that is everywhere uni- form; but when they encounter some other bodies, they are subject to being deflected by them, or weakened in the same way as the movement descsrtes a ball or a stone thrown in the air is weakened by those bodies that it encounters; for it is quite easy to believe that the action or inclination to move, which I have said light must be taken to be, must follow in this the same laws as movement.

And it must be noted that the ball, aside from its simple and ordinary movement, which carries it from one place to another, can also have a second which makes it turn about its center, and that the speed of this latter can have many different proportions with the speed of the former. Now, when many balls, coming from the same direction, encounter a body whose surface is completely smooth and uniform, they reflect equally and in the same order, such that, if this surface is to- tally flat, they maintain the same distance between each other after having encountered it, that they had before; and if it is curved inward or outward, they will approach or move away from each other, more or less, in the same order, depending on the ratio of this curvature.

Here you see balls A, B, C figs. And, if these balls encounter an uneven surface, such as L or M fig. And they do not change anything besides this in the manner of their move- ment descarges its unevenness consists only in its parts being differently curved.

But it can also consist in many other things, and by these means can bring it about that, if these balls had earlier had only a simple rectilinear motion, they will lose a part of it and acquire instead a circular motion, which can 4 Descartes held that a perfect rebound would involve no change in speed: Those who play tennis prove this sufficiently when their ball encounters uneven ground, or when they hit it obliquely with their racket, which they call, I believe, cutting or grazing.

Finally, consider that, if a moving ball encounters obliquely the surface of a liquid body through which it can pass more or less easily than dioptrixs that which it is leaving, it is deflected and changes its course when it enters: Among these latter, some cause rays to reflect without causing diopttrics other change in their action, namely those that we call white; and others bring with this reflection a change similar to that received by the movement of a ball when diioptrics is grazed, namely those which are red, or yellow, or blue, or of any other such color.

For I think it possible to determine what the nature of each of these colors consists of, and make it known by experiment; but this goes beyond the bounds of my subject.

As, even though those which fall upon the surface of a white body AB fig.

DESCARTES – Dioptrics-On Light | Thiago Lédo –

And dezcartes same is true, if we assume this body to be quite thin, like a descartee of paper, or a cloth, such that light passes through it, even though the eye dezcartes on the side opposite the flame, such as towards E, some rays of each of the parts of the body will still be reflected towards it. Finally, consider that the rays are also deflected in the same way as was said of a ball when they encounter obliquely the surface of a transparent body through which they penetrate more or less easily than through the body from which they came, and in these bodies this manner of being deflected is called refraction.

Second Discourse On Refraction Inasmuch as we will later need to know the quantity of this refraction exactly, and since it can be understood easily enough by the comparison which I have just used, I believe that it is appropriate that I try here to explain it all at once, and that I first speak of reflection, in order to make the understanding of refraction dioptrixs much the easier.

But in order not to be tripped up by new difficulties, let us assume that the ground is completely flat and solid, and that the ball always has an equal speed, both in descending and in ascending, without inquiring into the power which continues to move it after it is no longer touched by the racket, nor shall we consider any effect of its weight, nor its size, nor its shape; for here there is no question of looking at it so closely, and none of these things are of relevance to the action of light, to which this inquiry must correspond.

Furthermore, it must be noted that the determination to move in one direction can, just as movement, and in general any sort of quantity, be divided into all the parts of which we imagine it is composed, and that we can easily imagine that the motion of the ball which moves from A towards B is composed of two others, one causing it to descend from the line AF towards the line CE, and the other at the same time causing it to go from the left AC to the right FE, such that these two combined direct it towards B along straight line AB.

And then, it is easy to understand that the encounter with the ground can only prevent one of these two determinations, and not in any way the other: Then, in order to know precisely to which among all the points of the circle it will return, let us draw three straight lines AC, HB and FE, perpendicular to CE, and in such a way that there is neither more nor less distance between AC and HB as between HB and FE; and let us say that in as much time as the ball has taken to advance to the right from A one of the points on the line AC to B one of the points on line HBit must also advance from line HB to some point on line FE: Now, it is a fact that it cannot arrive at the same time at a certain point on the line FE and at a certain point on the circumference of circle AFD, unless it is at point D or F, inasmuch as it is only these two where they [the line and the circumference] intersect each other, and since the earth prevents its going to D, it must be concluded that it indubitably goes towards F.


And thus you easily see how reflection occurs, to wit: Now, under this hypothesis, in order to know which path it must follow, let us consider afresh that its movement differs entirely from its determination to move more in one direction rather than another, from which it follows that their quantities must be examined separately; and let us also consider that of the two parts of which we can imagine this determination is composed, it is only that which makes the ball move from high to low that can be changed in some fashion by the encounter with the cloth, and that by which it is made to move towards the right always remains the same as it has been, because the cloth is in no way opposed to motion in that direction.

For, since it loses half its speed in passing through the cloth CBE, it must take twice as much time to pass below B to a point on the circumference of the circle AFD as it took above to pass from A to B: Then, as for the rest of the body of water that fills the entire space between B and I, although it resists the light more or less than did the air that we hypothesized earlier, this is not to say that it must divert it more or less: Apparently, anything that real balls do is considered foreign.

But let us make yet another assumption here, and let us consider that the ball, having first been impelled from A towards B fig. Finally, inasmuch as the action of light follows in this respect the same laws as the movement of this ball, it must be said that, when its rays pass obliquely from one transparent body into another, which receives them more or less easily than the first, they are deflected in such a way that they always 7 Yes, Descartes really is creating the image of a tennis racket appearing out of nowhere to whack the ball downwards as it enters the water!

Only, care must be taken that this inclination be measured by the quantity of straight lines, like CB or AH, and EB or IG, and similar lines, compared one with the other, rather than being measured by the quantity of the angles, like ABH and GBI, nor still less by the quantity of angles similar to DBI, which are named the angles of refraction. For the ratio or proportion which is between these angles varies at all the many inclinations of the rays, whereas that between lines AH and IG, or similar lines, remains the same in all the refractions caused by the same bodies.

It is good that now you see how different refractions must be measured; and although it is necessary to use experience to determine their quantities inasmuch as they depend on the particular nature of the bodies in which they occurwe are nonetheless able to do so reasonably certainly and easily, since all refractions are thus reduced to the same measure; for it suffices to examine them with a single ray to know all those [refractions] which occur at the same surface, and one can avoid all error, if several others are examined as well.

Dioptrique – Wikipedia

But perhaps you will be shocked while making these experiments, to find that the rays of light are more inclined in air than in water, on the surfaces where they refract; and still more so in water than in glass, quite contrary to a ball, which inclines more in water than in air, and cannot pass through glass at all: You will cease to this find strange, if you remember the nature that I at- diootrics to light, when I said that it is nothing other than descartex certain move- ment or an action, received in a very subtle material that fills the pores of other bodies; and if you would consider that, as a ball loses more of its ag- itation in running against a soft body than against a hard one, and that it rolls less easily on a carpet than upon a smooth table, thus the action of this subtle material can be much descaftes impeded by the dioptris of air, which are soft and poorly joined and do not make much resistance, than by those of water which give more [resistance]; and still more by those of water than by those of glass or crystal: Still, other bodies can be found, principally in the heavens, where refractions, proceeding from other causes, are not reciprocal in this way.

And certain cases can also be found where rays must curve, although they only pass through a single dioptics body; in the same way as the movement of a ball curves, since it is deflected towards one direction by its weight, and towards another by the action with which it descartees been impelled, or for many other reasons.

For, finally, I dare to say that the three comparisons which I have just used are so proper, that all the particularities which can be remarked about them correspond to others which are completely similar for light; but I have only sought to explain those which were the most relevant for my subject.

And I do not wish to make you consider other things here, except that the surfaces of transparent bodies which are curved deflect rays which pass through each of their points, in the same way as would flat surfaces that could be imagined touching these bodies at the same points: From this, you see that the rays can be brought together or descartess apart, accordingly as descarttes fall upon surfaces which are curved in various ways.

And it is time that I begin to describe to you the structure of the eye, in order that you will be able to understand how rays which enter into it dispose themselves there to cause the sensation of sight. Remember me on this computer.

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