NATURAL PHILOSOPHY. 137 



a shade of lighter green appears to spread over the whole figure, and 

 overlap the surrounding red ground. A red stripe upon a green ground, 

 when agitated, appears of a lighter red on each margin, alternately 

 \vith a deep-red wave, oscillating back and forth at each motion of the 

 card. It has been suggested that the pile of the worsted, in^ which 

 these figures are generally wrought, was essentially connected with the 

 phenomena. This Prof. Loom is had found not to be the case. He 

 had experimented with cotton, silk, leather, and paper ; and in all had 

 succeeded in producing an effect similar to that with the worsted. 

 AYith the colors of natural objects, he had found that the red of certain 

 flowers, combined with certain green leaves, exhibited the effect in 

 question very handsomely. The scarlet flower of the verbena, or 



feranium, combined with green leaves of lettuce, succeeds beautifully, 

 'hese experiments appeared to him sufficiently varied to prove that the 

 effect in question is entirely independent of the material employed. 

 Color alone appears to be the essential circumstance. A particular 

 shade of color is required, and also a certain intensity. A brilliant 

 red, combined with a complementary color, will always produce the 

 required effect. A lustre or gloss upon the surface interferes some- 

 what with the effect, but does not entirely destroy it. Such a result 

 might have been anticipated, because the existence of gloss implies 

 that foreign light, (which, of course, is not homogeneous,) is reflected 

 from the sin-face in question, and mingles with the light emitted from 

 the body under experiment. Of course the color of the body is changed, 

 or rendered less brilliant by this mixture. The conclusion arrived at 

 was, that the wave-like motion which passes over a small red figure, 

 upon a green ground, when gently agitated, is an effect of brilliant com- 

 plementary colors, and has no connection with the nature of the mate- 

 rial with which the color is associated. 



These phenomena are believed to involve the following principles : 

 1. The continuance of impressions on the retina. An impression made 

 on the retina lasts for an appreciable interval. "When a bright colored 

 figure is agitated before the eye, it makes an impression upon a portion 

 of the retina larger than that covered by the figure at rest. Thus a green 

 figure upon a red ground, being agitated, appears to overlap the sur- 

 rounding red ground. 2. There is a partial and transient combination 

 of the complementary colors. While the impression of the central 

 color remains upon the retina, the same portion of the retina, in con- 

 sequence of the motion of the card, receives a new impression of the 

 surrounding complementary color ; in other words, two colors, which 

 are complementary to each other, are impressed successively upon the 

 same part of the retina ; the new impression being made while the 

 effect of the old one remains. These two colors partially combine to 

 produce not white light but a much brighter shade of the primi- 

 tive color. This explains the experiment with the green figure on a red 

 ground ; and it explains the brighter shade on the margin of the red 

 figure upon a green ground. But the red stripe upon the green ground 

 exhibits the remarkable peculiarity of a deep red wave oscillating from 

 one side to the other of the strip at each motion of the card. In order 

 to analyze this phenomenon, I tried the following experiment: 



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