Geological Society, 483 



distance, do I see the object exactly defined on directing my eye to 

 it. On directing my eye to the further object, I see it, of course, 

 less defined than I do the nearer object when I direct my eye to it ; 

 but the difference is not at a glance very striking. 



This experiment must not be confounded with another adduced 

 by the late Professor Miiller as a proof of the existence of an adjusting 

 power in the eye. The experiment T refer to is as follows : — 



If we regard with one eye only (the other being closed) the ends 

 of two pins placed one before the other at different distances in the 

 line of the axis of the eye, one will be seen distinctly when the other 

 appears indistinct, and vice versd. Both images lie in the axis of the 

 eye, one over the other ; and yet it depends on a voluntary effort, 

 the exertion of which can be felt in the eye, whether the first or the 

 second pin shall be seen distinctly. " The two images of the pins," 

 says Miiller, " fall upon the same point of the retina; one lies over the 

 other, and yet I see the nearer through the cloud-like image formed 

 by the rays from the other more distant pin, and vice versd." 



If any person is able to see the phenomena here described, he is 

 undoubtedly endowed with an adjusting power in his eye. 



I have never succeeded in seeing the phenomena myself. 



In viewing objects at different distances, the sight is no doubt 

 aided by the movements of the eyebrows, eyelids, eyeballs, and pupils ; 

 but in this we have no example of adjustment properly so called, 

 viz. intrinsic adjustment. 



That the focal power of my eye may become slowly altered, for 

 instance by prolonged examination of near and minute objects, and 

 again slowly return to its former state, I am satisfied ; but this, again, 

 is no example of adjustment properly so called. 



P.S. It would oblige me very much, if any one into whose hands 

 this paper may happen to fall, and who may take the trouble to 

 analyse his sight in the manner herein described, would communi- 

 cate to me the results of his observation on the following points : — 



1. The distance at which the vertical line is seen with the greatest 

 distinctness and best definition. 



2. The distance at which the horizontal line is seen with the greatest 

 distinctness and best definition. — Or, 



3. If there be no difference in the distance at which the vertical 

 and horizontal lines are seen with the greatest distinctness and best 

 definition. — And, lastly, 



4. Whether or not the observer can satisfy himself that he has 

 the power of adjusting the eye, so as to be able to see the lines with 

 perfect distinctness and definition at any other than one distance. 



N.B. If spectacles are worn, mention the kind of glasses — whether 

 convex or concave — and their power. 



Note also if there be any difference in the sight of the two eyes. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 401.] 

 November 7, 1860. — L. Horner, Esq., President, in the Chair. 

 The following communications were read : — 

 1. "On the Denudation of Soft Strata." By the Rev. O. Fisher, 

 M.A., F.G.S. 



