SINGLE VISION. 37 



law of visible direction affecting each eye, when 

 employed by itself, in the same manner as when 

 it is used conjointly with the other*. 



* Du Tour expected, that if two objects of different colours 

 were seen in the same place by both eyes, which however he 

 says, he was never able to observe, the colour of the ap- 

 parently united object would be compounded of those of the 

 two really single objects. Memoires des Savans Etrangers, 

 torn. iv. p. 500. And Dr. Reid mentions expressly that it is 

 so compounded. Inquiry, p. 2Q3. But in all my experi- 

 ments upon this subject I have remarked, that, when the two 

 objects appeared united, each was seen, notwithstanding, in 

 its proper colour ; the red, for example, appearing as it were 

 through a transparent green, and the green, in the same ex- 

 periment, as through a transparent red. Nor is there any 

 thing in this inconsistent with the received doctrine of the 

 composition of colours. For in every instance of the pro- 

 duction of a new colour, from rays of different colours being 

 at the same time sent to the eye, these rays fall upon the 

 same sentient extremities of the same nerve. But, in the 

 case before us, the differently-coloured rays fall upon the 

 sentient extremities of two different nerves, which have no 

 communication with each other, except through the medium 

 of the brain. We have greater reason, therefore, for expect- 

 ing, that the colours impressed upon the two eyes, should be 

 perceived uncompounded, than there is for two colours being 

 perceived separately, which are impressed upon two different 

 parts of the same eye. 



From the fact of the two colours being thus perceived di- 

 stinct from each other, I would infer, by analogy, a mode of 

 argument indeed often fallacious, that if it were possible for 

 <as to hear any one sound with one ear only, and another 

 sound with the other ear only, such sounds would in no case 



