Refraction2

You see now how refractions should be measured; and although determining their quantity depends on the particular nature of the bodies in which they occur, and therefore requires experimentation, it can still be done with sufficient certainty and ease, now that they are all reduced to a single measure. For it suffices to examine a single ray to know all the refractions that occur in a given surface, and one can avoid all error by examining them further in a few other cases.

As if we want to know the quantity of those that occur in the surface CBR, which separates the air AKP from the glass LIS, we have only to test it in that of the rayon ABI, by seeking the proportion that is between the lines AH and IG. Then, if we fear to have erred in this experiment, it is necessary to test it again in a few other rayons, such as KBL or PRS, and finding the same proportion of KM to LN, and of PQ to ST, as of AH to IG, we will have no further occasion to doubt the truth.

But perhaps you will be surprised, * in performing these experiments, to find that the rays of light are inclined more in air than in water, on the surfaces where they are refracted, and even more in water than in glass, contrary to a ball which is inclined more in water than in air and cannot pass through glass at all.

For example, if a ball is pushed through the air from A to B and encounters the surface of the water CBE at point B, it will be deflected from B to V. However, if it is a ray of light, it will go from B to I, which you may find strange if you remember the nature of light that I have described, that it is nothing but a certain movement or action received in a very subtle matter that fills the pores of other bodies. And just as a ball loses more of its agitation when it hits a soft body than a hard one, and rolls less easily on a carpet than on a bare table, so the action of this subtle matter can be much more hindered by the parts of the air, which are soft and poorly joined, than by those of water, which make more resistance; and even more by those of water than by those of glass or crystal. Therefore, the smaller parts of a transparent body are harder and firmer, the more easily they allow light to pass through, because this light does not need to displace any of them from their places, unlike a ball that must displace those of the water to find passage among them.

Moreover, * knowing thus the cause of the refractions that occur in water and glass, and commonly in all other transparent bodies around us, one can remark that they must be all similar, when the rays exit from these bodies and when they enter them. Just as the rayon that comes from A to B is deflected from B to I as it passes from air into glass, the rayon that returns from I to B must also be deflected from B to A.

However, it is possible to find other bodies, mainly in the sky, where the refractions, proceeding from other causes, are not thus reciprocal. And it is also possible to find certain cases where the rays must bend, even though they pass through only a single transparent body, just as a ball curves under the influence of the wind, because it is deflected towards one side by its weight and towards another by the action that propels it, or for various other reasons. For indeed, I dare say that the three comparisons I have just used are so appropriate that all the particularities that can be observed in them relate to others that are equally similar in light; but I have only tried to explain those that were most relevant to my subject.

And I do not want to consider anything else here, except that the surfaces of transparent bodies that are curved deflect the rays that pass through each of their points, in the same way that flat surfaces, which one can imagine touching these bodies at the same points, would do. For example, the refraction of the rays AB, AC, AD, which come from the flame A and fall on the curved surface of the crystal ball BCD, must be considered in the same way as if AB fell on the flat surface EBF, and AC on GCH, and AD on IDK, and so on for the others.

Why the refraction of rays entering water is equal to that of rays exiting water. ** And why this is not general in all transparent bodies. *** That rays can sometimes be curved without exiting the same transparent body. **** How refraction occurs at each point on curved surfaces.

Figure # 15

From which you can see that these rays can be assembled or dispersed in various ways, depending on how they fall on surfaces that are curved in different ways. And it is time for me to begin describing the structure of the eye, so that I can explain how the rays that enter it are arranged inside to produce the sensation of sight.