i8o THE PHILOSOPHY 



fpace, at a certain diftance from the glafs, which is called the focus 

 of that lens. 



The white light conveyed to us by the fun is not homogeneous, 

 but confifts of feven differently coloured rays, or what are called^the 

 friinary colours. Thefe diff"erentiy coloured rays were difcovered by 

 Sir Ifaac Newton to have difl'erent degrees of refrangibility. When 

 the white light of the fun was made to pafs through a glafs prifm, 

 he found, that, inftead of retaining its original whitenefs, it exhi- 

 bited feven diftindt colours, and that this phaenomenon was produ- 

 ced by the feveral rays in the compofition of white light being more 

 or lefs refraded, or turned from their direit courfe. The fimple 

 primary colours are feven in number, namely, red, orange, yellow, 

 green, blue, indigo, and violet. Red is the leaft, and violet the moft 

 refrangible parts of white light. A proper mixture of all the feven 

 primary colours conftitutes whitenefs ; and by various combinations 

 of the primary colours, all the compound colours exhibited either in 

 Nature or art are produced. Any furface appears black when it re- 

 fi-eds little or no light. 



The different humours of the eye, and the cryftalline lens, are all' 

 denfer than air or water ; of courfe, their power of refracting the 

 rays of light is likewife greater. The rays proceeding from every 

 point of an objedl enter the pupil ; and the refradtion of the diffe- 

 rent parts of the eye, which adl as a lens, neccflarily makes them- 

 crofs each other in their paffage to the retina. After croffing, they 

 diverge till they are ftopped by the retina, where they form an in- 

 verted pidture. The upper part of the objedl is painted on the lower 

 part of the retina, and the right fide upon the left, &c. The cele- 

 brated Kepler firft difcovered, that diftincft, but inverted piftures of 

 every objedt we behold are painted on the retina by the rays of 

 ijght proceeding from vifible obje(ns. This difcovery naturally led 



Keplerj. 



