of the Eye in relation to the Perception of DlMance, 55 



suggests change of distance." I think that the muscular sen- 

 sation of convergence of the eyeballs serves rather to give an 

 idea of rate of change of distance * ; and I have just indicated 

 the cases to which variation of retinal magnitude are limited. 



24. In Wheatstone's classical research of 1838 it was de- 

 monstrated how great is the capacity of the brain to combine 

 together two slightly differing retinal images. This faculty 

 once admitted^ however explained, renders it unnecessary here 

 to discuss how far the inexactness of focus for any particular 

 tint is concerned in binocular vision. In the present paper it 

 must suffice to treat of the non-achromatism of the eye in its 

 relation chiefly to monocular vision. And, for reasons stated 

 in § 19, we shall omit further reference to linear perspective, 

 since its relation to our perception of distance is not based 

 upon apparent colour, or upon the formation of an exact 

 retinal focus. 



IV. Ej'.pe7'imental Results, and Deductions, 



25. Fraunhofer f is generally credited with the first careful 

 attempt to measure the chromatic aberration of the eye. His 

 method of experiment consisted in placing a wire in the 

 principal focus of a telescope, in w^hich the focus was then 

 obtained by the adjustment of the lenses of an achromatic 

 eyepiece. The theoretical adjustment to distinct vision for the 

 various coloured rays was then calculated and compared with 

 the actual adjustment ; and the difference betAveen theory and 

 observation was assumed to measure the aberration of the eye. 

 The method is at best only indirect ; and there appears to be 

 some doubt w^hether in the critical experiments an achromatic 

 object-glass as well as an achromatic eyepiece Avas employed. 

 Nevertheless, under the conditions of the experiment, the 

 measurements were very accurately made, and are therefore 

 invaluable. 



26. I have found the most satisfactory exhibition of 

 chromatic aberration to be manifested when a solution of 

 permnnganate of potash was employed, in a narrow glass 

 trough, as a medium to intercept the middle rays of the 

 spectrum, I have also tried solutions of iodine, and cobalt 

 glasses of various depths of tint ; but none are so satisfactory. 

 For some experiments I have employed a piece of thin cobalt 

 glass in conjunction with a solution of permanganate. By 

 this means I have obtained media transmitting red and blue- 



* See a curious little footnote by Mr. A, J. Ellis at p. 97 of liis edition 

 of Helmholtz's ' Sensations of Tone,' 



t See Gilbert's Annnlen, Ivi. 1814, p. 304; Helniholtz, Ph>isiol. Optik, 

 p. 128; Brewster, ^Eclin. Piiil, Journ/ ?vix. p. 25. 



