DALTONISM. APPARENT MOTION. 177 



and the eyes are fixed upon it for a time, a border of pale 

 green forms itself round the red ; and in the same way a yellow 

 square on a white ground produces a blueish crown round 

 the yellow image: these are called accidental fringes of light. 



M. Chevreul has discovered some remarkable laws which 

 govern the contrast of colours, and the mutual influence 

 which two colours placed in juxtaposition have upon each 

 other. The investigations of the eminent professor are not 

 more important for the arts than for science, for the pheno- 

 mena of irradiation are produced constantly in vision, and 

 artists should not forget them for a moment in painting or in 

 architecture. It is unnecessary to remark that the harmonious 

 or discordant effect produced by the association of colours in 

 these two arts is of the utmost importance, and although in 

 general the spectator troubles himself very little about the 

 law of contrasts, yet he is notwithstanding very sensible of 

 the impressions which result from its observance. 



Daltonism The effects of a disturbance in vision described 

 for the first time by an English chemist who was attacked 

 by it, are commonly designated by this term. It consists of 

 a difficulty, more or less great, of distinguishing colours, some 

 of which are entirely confounded although very different, as 

 rose and gray, red and green, &c. Very marked cases of 

 Daltonism are rarely met with, but in a slight degree the 

 affection is not uncommon. 



Apparent motion of objects. Among the most common 

 optical illusions we may cite those which consist of the 

 apparent motion of external objects. When on a boat, for 

 example, or in a carriage which is in motion, we seem to be 

 at rest while the shore or the sides of the road seem to be in 

 motion. We have no consciousness of the movement of 

 external objects except by being ourselves at rest, and when 

 the image of an object moves across the retina while the eye 

 and the body are in repose, the object seems to change its 

 position relative to us. Carried along by the boat or carriage, 

 without our bodies taking any active part in the movement, 

 we judge of the relative displacement instinctively, and from 

 habit we refer to external objects the movements which we 

 d-^ not ourselves feel. 



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