134 Miscellaneous Intelligence. 



jects themselves are withdrawn from view; and when a number of ob- 

 jects are presented to the eye in quick succession, the image left on 

 the retina by one object is liable to be confused by that of the suc- 

 ceeding object ; and when these are of different colors, a compound 

 color is perceived which is intermediate between the two; but as the 

 spectrum left by the first object is fainter than the image of that which 

 is actually present, it follows that the color of the latter will always 

 predominate. Hence in the blending of the two colors which takes 

 place at the junction of the spot and the ground of the pattern, the re- 

 sulting tints are different in each case ; for the red spot it is purple, that 

 is, the margin of the red spot is seen through a blue spectrum, that of 

 the ground, — while the border of the blue spot appears of a lavender 

 tint, the blue being seen through the spectrum of the red ground. 



From experiments made with glass of different colors, I venture to sug- 



gest this as the probable explanation ; for I find that red becomes darker 

 by being seen through a medium of light blue, while the blue is ren- 

 dered lighter by the aid of a red glass. 



These effects are not to be confounded with the phenomenon of " ac- 

 cidental colors," in which the spectrum of an object is of the opposite 

 or complementary color, these colors being produced only when the 

 eye has been exerted upon an object for some considerable time ; and, 

 moreover, the complementary colors of blue and red are not perceived 

 at all in these experiments. I mention this distinction, because an at- 

 tempt has been made to explain the apparent motion of these figures 

 by a reference to the theory of accidental colors. 



It remains to be shown why blue and red are the only colors to ex- 

 hibit this peculiarity. The only suggestion I have to make is, that the 

 intermediate tints formed by the admixture of blue and red contrast 

 strongly with both of the original colors, so as to produce the required 

 effects of lights and shade ; and certainly both purple and lavender 

 form decided contrasts with blue and red. And this is not the case 

 with any other combination of the primary colors. For example, the 

 orange tints produced by blending red and yellow contrast too feebly 

 with either of these colors for any effect of light and shade, and the 

 same objection applies to blue and yellow. 



Finally, why is the illusion seen only by artificial light? Not from 

 the blue being seen as green (because some degree of motion is ob- 

 servable in blue and red patterns by daylight if they are examined in 

 obscure corners), but from the indistinctness of artificial light as com- 

 pared with solar light ; for it will be found that if the colors are held 

 close to a lamp or candle so as to be strongly illuminated ; the effect is 

 destroyed ; and on the contrary, it becomes most striking in indistinct 

 lights, as when the patterns are held under cover of the table on which 

 the lamp stands, the shadows on the red spot then become exceedingly 

 dark, while the light tints on the blue spot have almost a phosphores- 

 cent appearance. For the same reason, when the pattern is viewed 

 indirectly, the effect is better than when the eye is directed full upon 

 it; and in patterns composed of several spots or figures on their appro- 

 priate ground, it is remarked that the apparent motion is greatest in 

 those parts on which the eye is not directed point blank, because our 

 vision is then indistinct. 



