TO DISTINCT VISION AT DIFFERENT DISTANCES. 6 



to produce a single end. I assume that each structural condition has a separate 

 end; the variable curvature to correct the aberration, the variable density to 

 alter the figure of it under pressure. 



2. The attempt to view near objects distinctly is accompanied by a sensible 

 muscular effort within the orbit. This is expressly stated, incidentally, by MtJLLER, 

 in a passage already quoted ; and has been admitted by every one whom I have 

 questioned on the subject. From my own sensation, I have no doubt that it is a 

 simultaneous effort of the four recti muscles drawing the eye back Avithin its 

 socket.* Such a retractive muscular action, fatal to the theory of elongation of 

 the eyeball, is just what we require to communicate to the fluid humours of the 

 eye, through the tough sclerotic coat in which they are bound, the hydrostatic 

 pressure which will act simultaneously upon all points of the crystalline lens. 



3. This theory is free from the unanswerable objections urged by Dr Young 

 and others, to all theories independent of that which ascribes the adjustment to 

 change of figure in the lens : and it is free from the objection to Dr Young's own 

 theory, which presumes a structure existing in the lens itself, unproved, and, to 

 say the least, improbable, — I mean its muscularity. 



4. It is confirmed by the fact, that where the lens is reproduced after the 

 operation for cataract, the power of adjustment is almost or totally lost; for, in 

 that case, it cannot be supposed that the new lens is provided with the requisite 

 gradation of coats for modifying its elasticity. 



5. The diminution of the adjusting power of the eye in old age is well ex- 

 plained by the collapse and induration of the lens, to the detriment of its elastic 

 properties.! 



6. That the crystalline lens is actually possessed of variable elasticity in dif- 

 ferent directions is rendered highly probable by Sir David Brewster's observa- 

 tions on its action on polarized light, in which tints are produced similar to those 

 in compressed jellies, and in minerals possessing different axes of elasticity. 



Postscript to the preceding Paper. 



When this paper was written, I had not seen Chossat's paper on the Forms 

 of the Refracting Surfaces of the Eye, which I have referred to. I have since read 

 it,f and find a remarkable confirmation of the views I entertain. 



It is very plain, that, were the gradations of the refrangibility of the coats of 



* I am aware that this is opposed to the experiment of Mr R.uhsden and Sir E. Home, which 

 seems to shew a protrusion of the eyeball. Supposing it correct, that protrusion must he equally the 

 result of muscular action producing pressure, dixe perhaps to the oblique muscles antagonising the 

 recti; for it is difficult to see where else it can be sought. 



t " In all animals the crystalline lens grows firmer with age." Brewster Edin. Encyc, Art. 

 Optics, p. 475. 



X In the Annales de Chimie, vol. x., published in 1819. 

 VOL. XVI. PART I. B 



