52 Mr. S. P. Thompson on the Chromatic Aberration 



power of accommodation of the eye to the requirements of the 

 mind in respect of retinal magnitude. Place under a stereo- 

 scope a penny and a halfpenny, having the " heads " of both 

 coins placed upwards and similarly. You shall see but one 

 coin when looking with the two eyes, especially if you expect 

 no dissimilarity and do not use the eyes alternately. There 

 are, on the other hand, experiments of Helmholtz, Fechner, 

 and Volkmann * which go to prove that the retinal magnitude, 

 as appreciated by the muscular sweep of the eye, affords a 

 fairly accurate means of comparison of the angular magni- 

 tudes and positions of two objects in the field of vision whose 

 images can be successively brought upon the same spot of the 

 retina. This involves, however, the question of the persistence 

 of visual impressions, without which such a comparison of re- 

 tinal magnitudes might be quite fallacious. 



17. The sensation (/) of tension of the ciliary muscle is a 

 very delicate means of estimating near distances, and is a 

 sensation capable of training to a considerable nicety. In 

 October 1870 the writer was unable to perceive any difference 

 of strain of adjustment to vision between distances of 4 yards 

 and 400 yards. At the present time he is conscious of a dif- 

 ference of tension when the eye is directed to objects 15 yards 

 and 300 yards distant respectively, and this with either eye. 

 I insist on this means of estimation of distance, because it is, 

 for monocular vision, of even more importance than the mus- 

 cular sensation of convergence of the optic axes is for binocu- 

 lar vision. 



18. The researches of Wheatstone, Brewster, Dove, Helmholtz, 

 Briicke, Miiller, Hood, Volkmann, and Wundt upon the stereo- 

 scopic presentation of objects resulting from the binocular dissi- 

 milarity of the two retinal images (^) illustrate the importance 

 of this fact of binocular vision. Wheatstone's pseudoscope 

 proved the point by a reductio ad ahsujxlum ; and an equally 

 conclusive proof is obtained by cutting a stereoscopic slide 

 in two and placing the halves in laterally reversed position 

 under the stereoscope. I have, however, experimentally found 

 that considerations of colour may sometimes outAveigh the 

 judgment founded by the eye upon the element of dissimi- 

 larity. I drew a pattern of two squares of four blue lines each 

 upon paper, and within each, and outside each, centrally de- 

 scribed circles in red. When this pattern was view^ed in the 

 stereoscope, the blue squares seemed to draw back from the 

 eye. I then traced some of Moser's crystal forms from the 

 well-known lithographed stereoscopic slides, colouring the 



* Helmlioltz, ' Popular Lectures,' p. 279 ; and Fhysioloyische Ojjtik, 

 pp. 622-524 and 542. 



