4 PROFESSOR FORBES ON THE ADAPTATION OF THE EYE 



followiiiff way : The crystalline lens, for example, that of the ox, is composed of a 

 nearly spherical nucleus of compact comparatively dense matter, of a hard pasty 

 consistence, which gradually, yet rapidly, passes into the gelatinous envelope of 

 a lenticular form, M'liich has far less consistence, and less resistance to external 

 pressure than the central spherule. It therefore occurred to me, that any nmform 

 pressure applied to the lens, such as might be communicated by the external 

 muscles of the eye to the entire eyeball, and propagated by hydrostatic pressure 

 through the humours, would tend to make the exceedingly flattoned ellipsoid of 

 the eye approach in figure to the dense spheroidal nucleus ; the obvious effect of 

 which would be, without any change in the position of the lens, to increase its 

 curvature, so as to render the rays from a near object more convergent. 



I proceeded, in April 1842, to endeavour to put my hypothesis to the proof, 

 bv subjecting the recent crystalline lens of a bullock to considerable hydrostatic 

 j)ressure, in a snitalile apparatus, and endeavouring to observe the change of focal 

 distance produced, making it act as the object-glass of a microscopic arrange- 

 ment ; but, partly owing to the difficulty of suspending the lens in a secure yet 

 free manner, partly from the unfavourable form of the glass vessel used for com- 

 pression, partly from the small excess of refracting power of the lens above that 

 of the Avater in which it was suspended, and partly from the essential indistinct- 

 ness of the picture formed in the dead eye, and the consequent difficulty of deter- 

 mining its precise focal distance ; — from all these causes my experiments failed 

 in yielding a positive result :* and though I communicated my vi(,»ws soon after to 

 Dr Alison, I, postponed any farther consideration or publication of the subject, 

 until I should be able to support the theory by decisive experiments. My attention 

 has been wholly diverted since to other inquiries ; and I see no prospect, at pre- 

 sent, of resuming the experimental part, which, no doubt, would be worth pur- 

 suit, and though difficult, is not I think, hopeless. In the mean time, the subject 

 of focal adjustment of the eye having been started at the late meeting of the 

 British Association at York by Sir D. Brewster, it occurred to me to state 

 verbally my notions; which having been thought worthy of attention, I have put 

 them into this more definite and permanent shape. 



In the absence of a direct proof in favour of my hypothesis (and this, it will 

 be observed, no other theory possesses), I may be allowed to state one or two cir- 

 cumstantial evidences in its favour. 



The first has been mentioned already, but is recapitulated for the sake of 

 connection. 



1. The crystalline lens possesses, on the common view, a twofold structure 



* It may be added, that the bullock's eye is perhaps one of the least favourable on which the 

 cxperiuicnt could be made. Owing to its very great convexity and thickness, it ma}' be presumed 

 that the action of compression above described will be much less visible than in a comparatively flat 

 lens, such as that of man. 



