CHRISTIAN HUYGHENS 53 



these profound subjects, and to lead us to hope them capable of 

 orderly explanation. Yet I have been surprised to find these very 

 investigators accepting arguments far from clear as if proof con- 

 clusive. No one has yet offered even a probable explanation of the 

 first two remarkable phenomena of light, — why it moves in straight 

 lines, and why rays from any and all directions can cross one another 

 without interference. 



I shall attempt in this treatise to submit clearer and more probable 

 reasons, along the lines of modern philosophy, first for the transmis- 

 sion of light, and, second, for its reflection when it meets certain bodies. 



Further, I shall explain the fact of rays said to undergo refraction 

 in passing through various transparent bodies. Here I shall consider 

 also, the refractions due to the differing densities of the atmosphere. 

 Later I shall investigate the remarkable refraction occurring in Ice- 

 landic crystals. Finally, I shall study the different shapes necessary in 

 transparent and reflecting bodies in order to bring together rays upon 

 a single point or to deflect them in different ways. Here we shall 

 see how easy it is by our new theory to determine not alone the ellipses, 

 hyperbolas, and other curves which M. Descartes has so shrewdly 

 constructed for this end, but as well the curve that one surface of 

 a lens must have when the other surface is known, as spherical, plane, 

 or any other figure. 



We cannot but believe that light is the motion of a certain material. 

 Thus when we reflect on its production, we discover that here on the 

 earth it is usually emitted from fire and flame, and that these unques- 

 tionably contain bodies in rapid motion, since they can soften and melt 

 many other more solid substances. If we note its effects, we see that 

 when light is brought to a point, as, for example, by concave mirrors, 

 it can cause combustion the same as fire : that is, it can force bodies 

 apart, a power that certainly argues motion, at least in that true 

 science where one believes all natural phenomena to result from 

 mechanical causes. Moreover, in my mind we must either admit this 

 or give up all hope of ever understanding anything in natural science. 



Since, according to this philosophy, it is believed certain that the 

 sensation of sight is produced only by the impulse of some form of 

 matter against the nerves at the base of the eye, we have yet another 

 reason for believing light to be a motion in the substance lying be- 

 tween us and the body producing the light. 



