252 



vergence of rays within the eye, it may save trouble to others to put 

 them on record. 



" The measures of the eye given in ahiiost every English work on 

 the subject, are those given by Young on his own authority, or that 

 of Petit. In the fifth volume of Dove's Repertorium, I find a series 

 of measures collected by Treviranus from his own and preceding ob- 

 servations, which I have converted below from French lines into deci - 

 nials of an English inch. In these the curvatures are supposed sphe- 

 rical. In the same work of Dove, I find a series of measures by Dr 

 Krause of Hanover, on eight recent human eyes, which seem to have 

 been made with uncommon care, and in which the deviation of the 

 surfaces from sphericity is noticed. I have preferred these last for 

 the purpose of calculation, because all the measures are taken from 

 the same eye, which is not the case with the numbers collected by 

 Treviranus. I have consulted the original paper of Krause in Pog- 

 gendorff's Annalen, vols. xxxi. and xxxix., where it appears, (1.) 

 That the cornea is thicker at the sides than in the centre ; (2.) 

 The anterior curve of the cornea is nearly spherical, the posterior 

 parabolic ; (3.) The anterior surface of the lens is elliptical, the lesser 

 diameter being in the axis of vision, the posterior surface is parabo- 

 lic ; (4.) The figure of the retina, or the posterior surface of the 

 vitreous humour, is an ellipsoid. 



" The following are those given by two eminent German authori- 

 ties, Treviranus and Krause, when reduced to English inches : — 



Mean of several Mean of eight 

 Authors by measures by 



Treviranus. Krause. 



Inches. Inches. 



Thickness of cornea (central part), . 0"032 0'040 



Distance of first surface of lens from back sur- 

 face of cornea, 



Pupil behind cornea, 



Thickness of lens, 



Axis of vitreous himiour. 



Axis of the eye from interior of the cornea to 

 the retina, .... 



Radius of first surface of cornea, 



Radius nf first surface of lens, 



of second do., 



Curvature of retina near the axis. 



These numbers agree tolerably well, only that the radius of cur- 



