$14 EXPERIMENTS ON DOUBLE VlSION. 



the point where we see them habitually. This allusion it 

 the more unavoidable, because a single perception always cor- 

 responds to a double image, and a double image is necessarily 

 produced by a single object seen by both eyes ; but in our 

 experiments, from the interposition of the vertical plane, 

 one impression only is received from each body, while with- 

 out this plane we should receive two impressions from each, 

 that is to say a quadruple image would be formed. 

 The combina- The apparent combination of heterogeneous colours is 



tion of colours another necessary consequence of this allusion, of this ap- 

 accounted for. ' . ■ ... • . . * 



parent displacement of the objects ; for we experience the 



sensation of mixed colours, as often as their united elements 

 produce their impressions conjointly on the organ of sight. 

 Thus a mixture of blue and red wool gives a violet colour 

 to the cloth made of it. Here, though the colours act 

 separately on each of the two eyes, it is on points, the 

 correspondence of which is so confirmed by habit, that 

 only one perception can result from them, which is con- 

 sequently composed of the different effects of the double 

 impression. 

 That of figures I say nothing of the phenomena of the association or 

 from the same comD i na (j on f figures, its theory flowing so naturally from 

 what has already been laid down, that a particular expla^ 

 Affinity of co- nation of it would be superfluous. As to the greater or 



lours simhar to j ess facility, with which heterogeneous colours combine by 

 that of concords •" ,. J 



in music. double artificial vision, i< is explicable in the same manner 



as the different effects of concords on the ear. In the same 

 manner as there are sounds, the association of which is 

 disagreeable, because their proportions are perceived with 

 difficulty, there are colours, the heterogeneousness of 

 which renders their association laborious, and consequently 

 not pleasing. I cannot discuss this question more amply, 

 without entering into the depths of the theory of sensations, 

 and wandering from my object, which was to make known, 

 . and to reduce to the common laws, some remarkable 

 facts, that add to the number of illusions we expe- 

 rience from the wonderful but not very accurate organ of 

 si^ht. 



Experiments 



