MEMOIRS. 



Observations on the Histology of the Eye. 

 By J. W. HuLKE, Esq., F.R.S. 



(With Plate XIX.) 

 The Apparatus of Accommodation. 



The term Accommodation. — Let me, in the first place, 

 endeavour to explain what the term accommodation techni- 

 cally means. 



That we cannot see perfectly distinctly at the same instant 

 two objects placed at different distances before the eye, is a 

 fact of the truth of which a moment's attention suffices to 

 convince the most unobservant person. The fact is most 

 easily realised when the objects are near, for when they are 

 at a great distance the minor distinctness of one of them is 

 less appreciable, but when they are relatively close to the 

 spectator it is impossible for him not to become aware of the 

 phenomena. Thus when I look at the nearer of two trees, 

 placed several yards apart, nearly in the same line, in a dis- 

 tant field, the minor distinctness of the further tree is so 

 slight that I may fail to notice it ; but when I look at a book 

 through a veil, both being near me and only a few inches apart 

 I find that when my eye is fixed on the print, I see it quite 

 distinctly, while I am scarcely conscious of the presence of the 

 intervening veil ; and again, when I look intently at the veil, 

 and perceive its texture distinctly, at that same moment the 

 print becomes confused and unrecognizable. What is the 

 explanation of this? In order to see an object distinctly an 

 exact image of it must be formed on the bacillary layer of 

 the spectator's retina. Every luminous point on the surface 

 of the object turned towards the spectator must be repre- 

 sented by a corresponding image upon his retina. This is 

 effected by the refractive poAver of the eye — that power its 

 transparent parts possess in common with inanimate trans- 

 parent bodies, of changing the original direction of the 

 luminous rays which enter it, and of giving those rays a new 



VOL. X. NEW SER. Y 



