in Single and Binocular Vision. 441 



the point a is made to pass into A, I have never succeeded in 

 making b pass into B. Whenever there is an appearance of 

 this, either turn round the paper, or the head, so as to sepa- 

 rate the lines as in fig. 6, and it will be invariably seen that if 



Fig. 5. Fig. 6. 

 A B a b A! B' 



y 



a springs out of A, ftwill spring out of a point between A and 

 B. The apparent coincidence, therefore, of A B with a b, 

 fig. 6, when it is seen, arises from the disappearance of one or 

 other of the extremities of the two lines. 



But Mr. Wheatstone has described another very interesting 

 experiment, of the same character as that which we have been 

 examining, and he regards it as "proving that similar pictures, 

 falling on corresponding points of the two retinae, may appear 

 double and in different places." Draw a strong vertical line, 

 A B, fig. 7, and another C D inclined some 

 degrees to it, and also a faint line m n parallel Q 

 to A B, and cutting C D at its centre S, then, 

 according to Mr. Wheatstone, the two strong 

 lines A B, CD, when seen with different eyes 

 in the stereoscope, or brought together by 

 looking at a nearer object, " will coincide, and 

 the resultant perspective line (CD) will appear 

 to occupy the same place as before ; but the 

 faint line (m n) which now falls on a line of the 

 left retina, which corresponds with the line of 

 the right retina, on which one of the coinciding 

 strong lines, viz. the vertical one (A B) falls, 

 appears in a different place." In repeating 

 this experiment, I have occasionally observed 

 an apparent coincidence similar to that which is described in 

 the preceding passage; but after numerous and varied obser- 

 vations, made with lines coloured and uncoloured, opake and 

 transparent, similar and dissimilar both in strength and form, 

 I have no hesitation in affirming that the pha?nomenon de- 

 scribed by Mr. Wheatstone is an illusion, arising from the 

 actual disappearance of one or more parts, or even of the whole 

 of one of the lines, and from the difficulty of observing the 

 separation or superposition of images in the circumstances 

 under which the experiment is made. 



The following are a few of the variations of the experiment 

 which I have found the best calculated to exhibit the real 

 place of the combined images. 



1. In Mr. Wheatstone's form of the lines shown in fig. 7, 

 the strong line A B assumes more readily the appearance of 



