52 Mr Smith on a Singular Phenomenon in Vision. 



the consequences which have been deduced from experiments 

 hitherto made on the intensity of sounds propagated in differ- 

 ent media, because the change in the mode of vibration of the 

 sounding bodies has not been taken into account. From what 

 has been above stated, it is easy to conceive, that the only way 

 to render experiments of this kind comparable, would be to 

 make them with long and thin bodies, to which are communi- 

 cated a tangential longitudinal motion, the only case in which 

 the number of vibrations cannot be influenced by the action 

 of different media. 



Art. X. — Account of a Singular Phenomenon in Vision. By 

 Mr Thomas Smith, Surgeon, Kingussie. * In a Letter to 

 the Editor. 



Sir, 

 On the 16th of February last, I was repeating with candle- 

 light some experiments which I had made before with the 

 light of day, to observe in what degree the sensation which a 

 luminous object, seen by both eyes on corresponding points of 

 the retina, differs from that which is produced, when it is seen 

 by both eyes on points of the retina not corresponding. I 

 held a slip of white paper perpendicular to the horizon, about 

 a foot from my eyes, and directing them to an object at some 

 distance behind it, saw, of course, two images of the white 

 paper. I was surprised, however, to find that the colours of 

 these two images were not the same, and neither of them white 

 like the slip of paper ; but that, on the contrary, they were 

 complementary red and green, so that, when, by changing the 

 direction of my eyes, I caused the two images to coalesce in 

 the middle, the resulting colour was white like the paper 

 viewed. For a moment I suspected that these appearances 

 arose from a sudden morbid affection of my eyes, for, though 

 I had often repeated the same experiment before, I had never 

 observed that the colours of the two images were different. 

 However, as in this experiment, the candle stood only a few 

 inches from my right eye, so that it was strongly acted upon 

 by the light, while the left eye was entirely shaded from it ; 



* Read before the Royal Society of Edinburgh on the 3d April 1826. 



