Mr Smith on a Singular Phenomenon in Vision. 53 



and as I was not ignorant that the action of strong light on 

 one part of the retina appears to affect the sensibility of the 

 surrounding parts, I thought of trying if that circumstance 

 had any share in producing the phenomena. I therefore 

 shifted the candle from the right to the left side, placing it so 

 that it might be seen by the left eye, but not by the right. 

 Instantly the colours of the two images were reversed, that 

 which was green before being now red, and that which was 

 red before appearing now green ; the paper always appearing 

 green to the eye on which the direct light of the candle fell, 

 and red to that which was in the shade. 



At my request, several other persons, both old and young, 

 repeated the same experiment, and, without knowing the re- 

 sult I had obtained, reported unanimously, that, of the two 

 images of the white paper, that which was nearest to the can- 

 dle appeared red, and the other green, or, as some termed it, 

 blue, and that, when the images coalesced, the mixture of the 

 two colours appeared white. 



I varied the experiment, by employing slips of paper of 

 different colours. When light red was used, the image seen 

 by the eye, acted on by the light of the candle, appeared 

 nearly white, and the other deep red. When faint green paper 

 was employed, the shaded eye saw it nearly white, and to the 

 other it appeared a stronger green. 



As some persons may find a difficulty in attending to the 

 two images while the eyes are directed to a distant object, 

 that inconvenience may be remedied, and the same results ob- 

 tained, by directing both eyes to the slip of paper itself, and 

 pressing the side of one of the eye-balls. This, as is well 

 known, produces two images of the object ; and if the light is 

 properly placed, one of these images will be seen red, and the 

 other green. 



When two candles were used, and so placed on each side, 

 that the light of the one acted only on one eye, and that of the 

 other on the other eye, the images of a slip of white paper 

 appeared white, if the two lights were equal, and at equal 

 distances from the eyes. But if the lights were unequal, or 

 at unequal distances, the two images appeared of different 

 colours; a fact which might perhaps furnish a method of 



