4 PROFESSOR FORBES ON THE ADAPTATION OF THE EYE 
following 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, which has far less consistence, and less resistance to external 
pressure than the central spherule. It therefore occurred to me, that any wniform 
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 flattened 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, 
by subjecting the recent crystalline lens of a bullock to considerable hydrostatic 
pressure, in a suitable 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 water 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 views soon after to 
Dr Attson, 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 
experiment could be made. Owing to its very great convexity and thickness, it may 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. 
