ikct. in] DISSERTATION SECOND. 79 



perpendicular to the surface is increased by the attraction 

 of the body, and, according to the principles of dynamics 

 (the 39th, Book I. Princip.), whatever be the quantity of 

 this velocity, its square, on entering the same transparent 

 body, will always be augmented by the same quantity. But 

 it is easy to demonstrate that, if there be two right-angled 

 triangles, with a side in the one equal to a side in the other, 

 the hypothenuse of the first being given, and the squares of 

 their remaining sides differing by a given 6pace, the sines of 

 the angles opposite to the equal sides must have a given 

 ratio to one another. 1 This amounts to the same with say- 

 ing, that, in the case before us, the sine of the angle of inci- 

 dence is to the sine of the angle of refraction in a given 

 ratio. The explanation of the law of refraction thus given 

 is so highly satisfactory, that it affords a strong argument in 

 favour of the system which considers light as an emanation 

 of particles from luminous bodies, rather than the vibrations 

 of an elastic fluid. It is true that Huvgens deduced from 

 this last hypothesis an explanation of the law of refraction, 

 on which considerable praise was bestowed in the former 

 part of.this Dissertation. It is undoubtedly very ingenious, 

 but does not rest on the same solid and undoubted princi- 

 ples of dynamics with the preceding, nor does it leave the 

 mind so completely satisfied. Newton, in his Principia, has 

 deduced another demonstration of the same optical proposi- 

 tion from the theory of central forces. 2 



The different refrangibility of the rays of light forms no 

 exception to the reasoning above. The rays of each par- 

 ticular colour have their own particular ratio subsisting be- 

 tween the sines of incidence and refraction, or in each, the 

 square that is added to the square of the perpendicular 



1 Optics, Book II. Part iii. prop. 10. 



3 Prin. Math. Lib. T. prop, 94. Also Optics, Book I. prop. 6. 



