TO DISTINCT VISION AT DIFFERENT DISTANCES. 3 



wise probable opinion is contradicted by the fact, that the muscularity of the 

 lens is unproved, and that this organ is wholly unprovided with bloodvessels 

 and nerves. The opinion now adopted by several eminent living authors is, that 

 the " first step in the process is the valuation of the pupil, which seems, by a 

 mechanism at the base of the iris, to increase the distance of the lens from the 

 retina."* This is very vague ; it is shewn by conclusive experiments that the 

 simple contraction and expansion of the ms produces no effect on the focal ad- 

 justment ;t and it is a mei-e conjecture that any of the organs connected with 

 the iris, the ciUary body for instance, has, or can have, any influence in puUing 

 the lens forward from the retina in any degree, much less through the consider- 

 able space requisite. We may, therefore, accept the resume of a late French writer 

 on Physics, as nearly expressing the opinion of the most candid authors upon 

 this vexed subject : " Tout cela n'est pas tres-satisfaisant, et il faut avouer que 

 I'explication de la nettet^ de la vision a des distances si differentes est encore a 

 trouver." 



Such being the present phase of the question, the suggestion of a " possible 

 explanation" yet unthought of, of the manner of the adjustment of the eye, may 

 be received with indulgence, or at least proposed without presumjjtion. 



About three years ago, whilst lecturing on the subject of vision, I was struck 

 with the circumstance, that the crystalline lens possesses not only a remarkable 

 gradation of consistence or density from the centre towards the surface, and espe- 

 cially towards the edges, whereby, according to the common explanation, the 

 spherical aberration of the rays of light is completely corrected ; but likewise a 

 complex and singular figure, which it is plain might alone produce the same 

 eifect by the modified curvature of the surfaces. Here, then, we appear to have 

 two peculiarities of structure to attain one end ; and it seems so natural, that 

 the curves should be proper curves for destroying the aberration of sphericity, 

 instead of the spherical cm-ves which are used in our instruments only from our 

 incapacity to form better ones,| that it occurred to me that the remarkable vari- 

 ations of density in the lens must be intended to answer another purpose. 



This purpose I conceived might be the focal adjustment, and effected in the 



* Brewster in Art. Optics, Encyc. Brit. 7th Edit. p. 513. 



t See MiJLLER and Brewster. 



I The forms of curvature of the crystaUine lens are said to have been actually ascertained by 

 M. Chossat to be ellipsoidal. It is a curious proof of the vagueness with which this subject has been 

 treated, that, in the clear and able work of Professor Lloyd on Light and Vision, in one page, the form 

 of the surfaces is insisted on as the means of producing distinct vision ; and on another, the gradation 

 of density from the centre to the side of the lens ; whereas, it is certain, that if the compensation for 

 spherical abeiTation due to the last cause be correct, the ellipsoidal form will be erroneous. Thus, as 

 in many other cases, the argument for design has been made to prove too much. See Lloyd on 

 Light and Vision, pp. 264-266, who refers to Chossat's paper, Ann. de Chemie, vol. x. 



