202 



EXPERIMENTS ON DOUBLE VISION. 



well as when they are homogeneous ? Such is the question 

 I propose to resolve. I confess, however, that I was led 

 to the experiments, that constitute the subject of this me- 

 moir, by an accidental circumstance, on occasion of an 

 eclipse of the sun, the progress of which I set myself to 



An eye viewing observe. The instrument I used not being furnished with a 

 an eclipse with- ° 



out a dark glass, coloured glass, my right eye was so much affected by it, 

 its sight was th a t ^ was deprived of distinct vision for some days. When 



destroyed for a . * " 



time, and after- it began to recover, all white objects appeared to me to 



wards, repre- have changed colour, and acquired a reddish hue, the 



sented objects , 



red. depth of which I could alter at pleasure, by looking at 



them with both eyes, or with the affected eye alone. When 

 This blended I looked at them with the right eye, which had been over- 

 ral hue when strained, they had a red hue; when with both eyes a rose co- 

 botk eyes were lour ; when with the left eye only, white as usual. This fact, 

 the explanation of which is foreign to my subject, led me 

 to conclude, not only that the perception produced by the 

 impulse of homogeneous light on one of the eyes was re- 

 inforced by that from a similar impulse on the other ; but 

 that the impression of heterogeneous rays on each of the 

 two eyes might give birth to a complex perception, which, 

 being composed of both sensations, would be a mean be.^ 

 tween the two. 

 Experiments on Desirous of satisfying myself whether the impression of 

 loursTnterposed a ^ ^ e P r * m itive colours, applied separately and simultane- 

 between the ously to both eyes, would constantly produce a complex 

 or/ect sensation analogous to that I have just related, I determined 



to procure myself transparent coloured mediums, which, 

 suffering rays of one sort only to pass, might, by being ap- 

 plied separately to each eye, subject this double organ to a 

 complex impression. The difficulty of procuring myself 

 coloured glasses of all the tints, or colours sufficiently trans- 

 parent to paint similar ones to those of magic lanterns, in- 

 duced me to reject these, which would have been more 

 convenient, and have recourse to hollow quadrilateral prisms 

 of white glass, into the cavity of which I poured liquids of 

 a proper colour and tint for all my experiments. 

 Choice of co- The choice of tingeing substances for colouring the wa- 

 lounng sab- ter, with which the prisms were filled, requires some pre- 

 cautions, of which it may be proper to inform the reader ; 



not 



