258 Prof. Wheatstone on the Physiology of Vision. 



to recede from the eye while the other remains at a constant 

 distance ; this is effected merely by pulling out the sliding board 

 C (fig. 8) while the other C remains fixed, the screw having pre- 

 viously been removed. 



Though the single appearance of two images of different size 

 is by this experiment demonstrated, the observer is unable to 

 perceive what difference exists between the apparent magnitude 

 of the binocular image and that of the two monocular images ; 

 to determine this point the stereoscope must be dispensed with, 

 and the experiment so arranged that all three shall be simulta- 

 neously seen ; which may be done in the following manner : — 

 The two drawings being placed side by side on a plane before 

 the eyes, the optic axes must be made to converge to a nearer 

 point as at fig. 4, or to a more distant one as at fig. 3, until the 

 three images are seen at the same time, the binocular image in 

 the middle, and the monocular images at each side. It will thus 

 be seen that the binocular image is apparently intermediate in 

 size between the two monocular ones. 



If the pictures be too unequal in magnitude, the binocular 

 coincidence does not take place. It appears that if the inequal- 

 ity of the pictures be greater than the difference which exists 

 between the two projections of the same object when seen in the 

 most oblique position of the eyes (i. e. both turned to the extreme 

 right or to the extreme left), ordinarily employed, they do not 

 coalesce. Were it not for the binocular coincidence of two 

 images of different magnitude, objects would appear single only 

 when the optic axes converge immediately forwards; for it is 

 only when the converging visual lines form equal angles with the 

 visual base (the line joining the centres of the two eyes) as at 

 fig. 2, that the two pictures can be of equal magnitude; but 

 when they form different angles with it, as at fig. 24, the di- 

 stance from the object to each eye is different, and consequently 

 the picture projected on each retina has a different magnitude. 

 If a piece of money be held in the position a (PI. VII. fig. 24), 

 while the optic axes converge to a nearer point c, it will appear 

 double, and that seen by the left eye will be evidently smaller 

 than the other. 



§ 14. Phenomena which are observed when pictures, which are 

 neither similar nor the binocular complements of each other, are 

 simultaneously presented to corresponding parts of the two retina. 



If we regard one picture with the right eye alone for a con- 

 siderable length of time it will be constantly perceived ; if we 

 look at the other with the left eye alone its effect will be equally 

 permanent; it might therefore oe expected that if each of these 



