2<JS MECHANISM OF 'JHE EYE, $ZC, 



The refractive denflty, in the fenfe of thefe proportions, varies* 

 as the ratio of the lines, and as the velocity of light in the medium. 



(Schol. 2. Prop. I.) Let the velocity at the diftance x be x r j 

 then, coniidering the refra6live force as a fpecies of attraction, wc 



have, in Prop. 41. 1. 1. Princip. v'ABFD = x r , Qj= S) the 



— -i 



fine of incidence, the radius being unity, Z zz sx ,Df 





2 XX \/ -^- 1 * x -*= l i s * • 1 -'" / ' . *»d 



I 



z 



the fluxion of the area defcribed by the radius zz — I s x 



. w 



X . 1 — j 2 x . Let the fine of the inclination to the radius 



at each point be called j; ; then y zz s x , .v zz j* 



^r, and the fluxion of the area zz y • 1 "-~yy\ 2 > of which 



r 



the fluent is — -^- Y,_y being the fine of the arc Y ; and the angle 



correfponding is Y. The value of that angle being found for 



any two values of xory, the difference is the intervening angle de- 

 fcribed by the radius. This angle is therefore always to the dif- 

 ference of the inclinations as r to r — 1, and the deviations is to 

 that difference as 1 to r — 1 . 



Corollary. Hence, in the paffage to the apfis, and the return to 

 ♦he furface, the deviation is always proportionate to the arc cut off 

 by the incident ray produced : therefore fuch a fphere could never 

 collect: parallel rays to any focus, the lateral denfity being too 

 fmall towards the furface. 



Page 259, line 9, for but the two laft, &c. read the feventh may 

 either be deduced from the eighth, or may be demonftrated inde- 

 pendently of it. 



Page 264, laft line, after internally, infert Or, if a lens of equal 

 mean dimenfions, and equal focal length, with the cryftalline, be 

 fuppofed to confift of two fegments of the external portion of fuch a 

 fphere, the refractive denfity at the centre of this lens mull be as 

 18 to IT. 



XI. Experiment* 



