﻿of Binocular Vision. 137 



distance of two feet from the object 1 obtained the image very 

 clearly and without much difficulty; but on approaching to 

 within ten or twelve inches it was only by patient trial for some 

 time that it could be brought out with perfect distinctness. 

 When the object was twelve inches from the eyes, the image, by 

 calculation, was found to be about thirty inches distant. By 

 turning the diagram so that the diagonals were horizontal, and 

 similar points therefore more than two inches apart, the image 

 was seen at the distance of about six feet. It had the exact ap- 

 pearance of a tessellated marble pavement made up of squares 

 nine inches on a side. 



In all these experiments the least irregularity in the pattern 

 shows itself very conspicuously in the image, not by indistinct- 

 ness of outline of the figures, but by apparent inequality in the 

 plane of the image. Thus in the carpet it shows itself by an 

 apparent wrinkle, in the lined diagram by some of the lines 

 rising like black threads stretched above the general surface of 

 the image. This phenomenon is a familiar one in stereoscopy, 

 and is used for detecting the slightest difference in two appa- 

 rently similar patterns, as, for instance, between a genuine and 

 a forged bank-bill. 



I believe any one, and particularly any young person with 

 good eyes, can with practice succeed in all the experiments de- 

 tailed above. Several of my family have tried them with success. 

 Yet in all cases it requires some practice to succeed well. I can, 

 even yet, always detect some difficulty on first trial after an in- 

 terval of a few days. But after several hours* practice the illu- 

 sion is so complete that it is almost impossible to dispel it. The 

 image is so real, that in attempting to recover the real object 

 by relaxing the convergence of the optic axes, the doubling of 

 the lines causes the eyes instinctively to return to their former 

 position, and thus to restore the image. I have sometimes 

 been actually obliged to look away in order to recover the real 

 object. 



The experiments detailed above have an important bearing on 

 some points in the theory of vision. It is the universally ac- 

 cepted doctrine among physiologists that the axial and focal ad- 

 justments of the eye cannot be dissociated. Helmholtz, speaking 

 of the consensual movements of the eyes, says, " We cannot 

 turn one eye up and the other down ; we cannot move both 

 eyes at the same time outward; we are obliged to combine 

 always a certain degree of accommodation of the eye to distance 

 [focal adjustment], with a certain angle of convergence of the 

 axes [axial adjustment]"*. He proceeds, however, to give cer- 

 * Helmholtz, Croonian Lecture, Proc. Koy. Soc. April 1864. 



