Prof. Wheatstone on the Physiology of Vision. 259 



pictures were presented to its corresponding eye at the same time 

 the two would appear permanently superposed on each other. 

 This, however, contrary 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, whde the 

 letter within it will change alternately from that which would be 

 perceived by the right eye alone to that which would be per- 

 ceived 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 appear mingle with them, and are 

 immediately after replaced by the entire letter. It does not ap- 

 pear 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 alternations 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 generally 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 different colours are presented to corresponding parts of the 

 two retina?. 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 two colours had min- 

 gled before their arrival at a single eye, the miud will perceive 

 the two colours distinctly, one or the other alternately predomi- 

 nating either partially or wholly over the disc. In the same 

 manner the mind perceives no trace of violet when red is pre- 

 sented to one eye and blue to the other, nor any vestige of orange 

 when red and yellow are separately presented in a similar man- 

 ner. These experiments may be conveniently repeated by placing 

 the coloured discs in the stereoscope, but they have been most 

 usually made by looking at a white object through differently 

 coloured glasses, one applied to each eye. 



In some authors we find it stated, contrary to fact, that if 

 similar objects of different colour be presented one to each eye, 

 tin- appearance will be that compounded of the two colours. 

 Dr. Ueid* and Janin are among the writers who have fallen into 

 tin- error. 



* Knquirv, Suet. xiii. 



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