100 ON THE APPARENT riGUKE OF STARS. 



Theophilus asserts, that the interior surface of the crystal- 

 line IS less convex than the exterior : fig. 10, Fallopius, Zinn^ 

 and many other anatomists, are of the same opinion, 



Vassalli says, he has observed the convexity equal on both 

 surfaces : fig. 11. 



Brisseau asserts, that the surface of the crystalline next the 

 cornea is more convex, than that which is in contact with the 

 vitreous humour. 



Petit, who had seen and observed a great number of crystal- 

 lines, says he observed some to have the anterior curvature 

 greater than the posterior, fig. 12; but that most commonly 

 the side toward the cornea had a radius of curvature greater 

 than that in contact with the vitreous humour, fig. 10, He 

 says even, that he found in several subjects the crystallines to 

 have their greatest curvature, one at the anterior surface, the 

 other at the posterior j and that he has met with crystallines 

 both surfaces of which were equal, fig. 11. 



Those anatomists a^ree in considering the two surfaces of 

 the crystalline as two segments of a sphere applied to each 

 other, fig, 13, 



Dr. Thomas Young concludes from observations made on 

 his own eyes, that the anterior surface of the crystalline must 

 be a portion of an hyperboloid, and the posterior surface a por- 

 tion of a paraboloid : but he admits, that his experiments 

 would not succeed equally with every eye. 



Dr. Petit says, he has met with crystallines, the posterior 

 convexity of which was not spherical, but approached the para- 

 boloid form. 

 Its figure varies -^'^ persons, who have made observations on crystallines, 

 with age. know that their figure varies with age. Those of infants are 

 small and thick. In some foetuses the thickness is but little 

 less than the breadth. Those of adults are about twice as 

 broad as they are thick. Those of old men grow flatter and 

 yellow. 

 Tt» dimension!. Petit has observed, that the dimensions of the crystalline are 

 not always proportional to the age ; and though in general the 

 breadth is 9 mil. [3 54 lines],and the thickness 4*5 [177 lines], 

 he has found it from 5 to 8 thick [r97 to 3*15/,], and even 9 

 [8-54/.]. 



Thus, 



