386 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



take place. It appears that if the inequality of the pictures be greater than the dif- 

 ference 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 ex- 

 treme 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 con- 

 verging visual lines form equal angles with the visual base (the line joining the cen- 

 tres 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 distance 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, (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 objects of different Jorms are simultaneously 

 presented to corresponding parts of the two retince. 



If we regard a picture with the right eye alone for a considerable length of time it 

 will be constantly perceived ; if we look at another and dissimilar picture with the 

 left eye alone its effect will be equally permanent ; it might therefore be expected, 

 that if each of these pictures were presented to its corresponding eye at the same time 

 the two would appear permanently superposed on each other. This, however, con- 

 trary to expectation, is not the case. 



If a and b (fig. 25.) are each presented at the same time to a different eye, the 

 common border will remain constant, while the letter within it will change alternately 

 from that which would be perceived by the right eye alone to that which would be 

 perceived by the left eye alone. At the moment of change the letter which has just 

 been seen breaks into fragments, while fragments of the letter which is about to ap- 

 pear mingle with them, and are immediately after replaced by the entire letter. It 

 does not appear to be in the power of the will to determine the appearance of either 

 of the letters, but the duration of the appearance seems to depend on causes which 

 are under our control : thus if the two pictures be equally illuminated, the alterna- 

 tions appear in general of equal duration ; but if one picture be more illuminated than 

 the other, that which is less so will be perceived during a shorter time. I have gene- 

 rally made this experiment with the apparatus, fig. 6. When complex pictures are 

 employed in the stereoscope, various parts of them alternate differently. 



There are some facts intimately connected with the subject of the present article 

 which have already been frequently observed. I allude to the experiments, first made 

 by Du Tour, in which two diflferent colours are presented to corresponding parts of 

 the two retinae. If a blue disc be presented to the right eye and a yellow disc to the 

 corresponding part of the left eye, instead of a green disc which would appear if these 



