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 variation 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 iris produces no effect on the focal ad- 
justment ;+ and it isa mere conjecture that any of the organs connected with 
the iris, the ciliary body for instance, has, or can have, any influence in pulling 
the lens forward from the retina in any degree, much less through the consider- 
able space requisite. We may, therefore, accept the reswmé 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 trés-satisfaisant, et il faut avouer que 
Yexplication de la netteté de la vision 4 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 presumption. 
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 
effect 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 curves 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 MiziEr and Brewster. 
t The forms of curvature of the crystalline lens are said to have been actually ascertained by 
M. Cuossar 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 Luoyp 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 aberration 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 Luoyp on 
Light and Vision, pp. 264-266, who refers to CHossat’s paper, Ann. de Chemie, vol. x. 
