Chap. 9.] Separation of component Rays, 26$ 



the fine of incidence in glafs be divided into fifty equal 

 parts, the fine of refraction into air of the leaft and 

 moil refrangible rays will contain refpectively 77 and 

 78 fuch parts. The fines of refraction of all the de- 

 grees of red will have the intermediate degrees of 

 magnitude, from 77 to 77-5-, orange from 77 v to 77!, 

 yellow from 77t to 777, green from 777 to 777, 

 blue from 777 to 77^, indigo from 77^ to 77^-, and 

 violet from 77^ to 78. 



According to the properties of bodies in reflecting 

 or abforbing thefe rays, the colours which we fee in 

 them are formed. If every ray falling upon an ob- 

 ject was reflected to cur eyes, it would appear white ; 

 if every ray was abforbed it would appear black ; be- 

 tween thefe two appearances innumerable fpecies of 

 colours may be formed by reflexion or tranfmiffion of 

 the various combinations of the colour-making rays *, 

 Jf the rays alfo of light were not thus compounded, 



every 



* The original or component rays of light are feparable from 

 each other, not only by refra&ion, or by varying the angle of in- 

 cidence on a reflecting furface, but are likewife at like incidences 

 more or lefs refleftible, according to the thicknefs or diftance be- 

 tween the two furfaces of the medium on which they fall. They arc 

 alfo liable to be turned out of their direct courfe by approaching 

 within a certain diftance from a body, by which means a fepara- 

 tion enfues, the rays being more or lefs deflected as they differ in 

 colour. Of thefe circumftances it will be proper to give fome 

 account. 



If a convex glafs or lens, or a portion of a fphere, is laid upon 

 another plane glafs, it is evident that it will reft or touch at one 

 particular point only ; and, therefore, that at all other places be- 

 tween the adjacent furfaces will be interpofed a thin plate of air, 

 the thicknefs of which will increafe in a certain ratio, according 

 to the diftance from the point of contact. Light incident upon 

 fuch a plate of air is difpofed to be tranfmitted or refle&ed ac- 

 cording to its thicknefs ; thus, at the center of contact, the light 

 is tranfmitted, and a black circular fpot appears j this fpot is en- 

 vironed 



