210 EXPERIMENTS ON DOUBLE VISION. 



Enumeration of The facts established by our experiments, and of which 

 the facts. we h ave £ gj ve ^ e p roo f ? may be reduced to the following. 



1. Simultaneous and separate double vision of objects 

 differing in colour produces a mixed or compound sensation, 

 which gives rise to a simple perception, similar to that 

 which would be produced by a mechanical mixture of co- 

 louring substances, representing the colours combined. 



2. Objects differing in figure, and similar in colour, com- 

 bine their figures, as objects of different colours do their 

 colours. 



3. Colours in their association, or apparent combination, 

 by double vision, seem to obey a kind of affinity, which 

 renders it more easy between some than between others. 



Reason why a The explanation of the first fact is deducible from the 

 simple percep- ommon ^ f our sensations. Perception beinsr eene- 

 tion is produced # _ . . 



by a compound rally proportional to the sensation, and this to the im- 

 9ensanou. pression made on our organs, a stronger impression must 



occasion a more lively sensation, and consequently a pro- 

 portional perception. Hence when an impression is double, 

 from being received by both eyes, it must be heightened. 

 The perception however will not be double, because we 

 distinguish similar impressions with difficulty; and under 

 the same circumstances, and by the same agents, we are sus- 

 ceptible of impression only to a certain degree. 



The facts respecting the apparent combination or asso- 

 ciation of objects of heterogeneous colours, by artificial 

 double vision, offer several questions to be solved with re- 

 Why do we not spect to their cause. The first and chief, with which all 

 see two objects ^ e {- ners are connected, is to know why, in these experi- 

 in this case, as . . 



■when both are ments, a double heterogeneous impression does not occa- 

 seen by each s j on a double perception, as when we see two objects with 

 both eyes at once ; and why on the contrary there is 

 but one perception, as when we see a single object with both 

 eyes. 

 Hypotheses of To explain this common phenomenon "of vision, physiolo- 

 physiologists gists have invented divers hypotheses, which I have at- 

 ons ' ngevis!0n ' tempted in vain to apply to the explanation of my experi- 

 ments. Some have asserted, that perception was simple in 

 Union of the consequence of a union of the optic nerves, which, being 

 optic nerves, dependant on each other in their functions, could therefore 



produce 



