ON THE APPARENT FIGURE OF STARS, ^9 



torn of the eye by the light traversing this organ, to approach so 

 much the more the form of a cross, in proportion as the kimi- 

 nous object is more remote from the eye, and as the two radii, 

 generating those surfaces, differ more from each other. 



The two ellipsoidal images decussating each other, fig. 8, are Ellipsoidal 

 constantly seen, if a ray of solar light, the light of a candle at ' 

 a distance, &c. be made to pass through an ellipsoidal lens. 

 The same image, too, is observable, if they pass through irregu- 

 lar surfaces ; such, for instance, as phials or decanters filled with 

 water, &c. 



It is extremely difficult to ascertain with precision the figure Figure of the 

 of the surface of the cornea in the living subject. After death comeadifficuk 

 the cornea becomes flaccid, and undergoes alterations that pre- to ascertain, 

 vent us from distinguishing accurately the nature of the sur- 

 face it had. Attempts were made to ascertain the figure of the 

 eye by freezing it : but the increase of bulk of the fluid by 

 congelation so altered it, that it was impossible to form a precise 

 idea of the nature of its curved surface. 



On looking at a fresh human cornea, it appears to be of an Apparent irre- 

 irregular figure, but this irregularity is occasioned, in great ^" ^"'^ '^ ' ' 

 measure, by the projection of the tunica conjunctiva over the 

 upper part of the cornea. Dr. Petit, who paid great attention Petit's opi- 

 to the figure and dimensions of the eye, says, in a paper pub- "'°"' 

 lished among those of the Academy of Sciences, in 1726, that, 

 when the portion of the conjunctive coat advancing upon the 

 cornea is dissected off, the latter is commonly round : yet he 

 met with the cornea of a negro, that measured 5 5 lines French 

 from right to left, and only four from top to bottom. I have 

 found it hitherto impossible, to obtain accurate data ; all my pj.q1,^Ij{„ „qj 

 obsers'ations, however, lead me to believe, that it is not sphe- spherical, 

 rical. 



As the crystalline may easily be separated from the eye, The crystal- 

 physiologists could not fail to make observations on it. Accord- ""^' 

 ingly, all who have treated on the organ of sight have been 

 ready to describe the figure and composition of the crystalline. 



Galen considers the crystalline as not being a perfect sphere, Different ac- 

 uniform throughout its whole extent ; but approaching to a com- counts of it$ 

 pressed globe, fig. 9. 



Rufus of Ephesus thinks, that from its figure it should be 

 called lenticular. 



H 2 ' The- 



