ON ACCIDENTAL OK SUBJECTIVE COLORS. 213 



attempt has been made to measure the time necessary for the complete pro- 

 duction of the impression. As regards the time during which a given impres- 

 sion preserves an intensity perceptibly constaiil^ M. Plateau has arrived at the 

 following conclusions : 



Laws of the 2^crsistence of images. — First. The interval of time during which 

 an impression is maintained without sensible loss is proportionably greater as 

 the impression is less intense. Second. This interval, for the production of a 

 complete impression on the eye of M. Plateau by a white paper, in broad day- 

 light, was found to be less than 0". 008 ; for yellow paper, a little longer than 

 white ; still longer for red, and longer still for blue. These limits may be much 

 exceeded when the impressions have little intensity. It will be seen that, ac- 

 cording to these results, it cannot be said in general, as in speaking of the par- 

 ticular case of a burning coal, that, in order to obtain a continuous sensation, it 

 suffices that the luminous impression be repeated eight or ten times per second. 

 In order to produce on the eye of M. Plateau the continuous sensation of white- 

 ness, such as that of a piece of paper in the full light of day, it was necessary, 

 from the number above given, that the impression of light should be repeated 

 more than 125 times per second. Third. The total duration of the impression 

 appears to be greater in proportion as the impression is more intense. When 

 the object which has produced the impression is very luminous, such as the set- 

 ting sun, and we cover the eyes, the impression sometimes subsists for several 

 minutes. In some persons, the imagination may exert great influence over phe- 

 nomena of this sort. Fourth, The total duration of the impression appears to be 

 greater in proportion as the object has been observed for a shorter time ; provided, 

 however, that the time be sufficient to develop a complete impression. When, 

 for example, M. Plateau cast his eyes on a small isolated window and speedily 

 directed them to an obscure part of the chamber, he saw the image with its 

 luminous panes and dark sashes distinctly persist, though growing fainter, for 

 about three seconds. If he looked at the window a little longer, the duration of 

 the image was less ; and, finally, after a prolonged observation of the window, 

 the image was so fugitiA'e that he could with difficulty perceive a trace of it. 

 Fifth. When the impression proceeds from a very luminous object, such as the 

 setting sun, or even a window, it usually passes through a series of different 

 colors. Thus, when, after having looked at a window for a very short time, M. 

 Plateau covered his eyes with a handkerchief, he saw the image of the luminous 

 panes become successively red, violet, blue, again violet, whitish, then greenish, 

 and green. These phenomena, which seem to vary much with the circumstances 

 of the experiment, have received as yet no satisfactory explanation. Sixth. 

 When, lastly, the impression persisting on the retina proceeds from a brilliant 

 object, it is observed occasionally to disappear and reappear several times in suc- 

 cession before it completely vanishes. This, for example, takes place, according 

 to Darwin, when the setting sun has been observed for some instants, so as not 

 too much to fatigue the sight, and the observer afterwards, with the eyes closed 

 and covered, considers the. persistence of the impression produced. 



Kaleidoplione. — Mr. W^heatstone has made use of the persistence of these im- 

 pressions to exhibit to the eye the mode in which the transverse vibrations of 

 an elastic rod attached by one of its extremities take place. For this purpose, 

 the rod is terminated by a small and polished metallic ball, or by a small hollow 

 sphere of glass, plated on the interior, and the apparatus is exposed to the sun 

 or the light of a candle. When the rod is put in vibration by means of a bow, 

 or by removing it from its position of equilibrium and then leaving it to itself, 

 the brilliant point which the sun or candle occasions on the little ball produces, 

 by its rapid movement, the appearance of beautiful curves, more or less com- 

 plicated, according to the form of the rod and the sound whichjt has been made 

 to produce ; thus each system of vibration and each sound has its image. To 

 this ingenious apparatus Mr. Wheatstone has given the name of kaleiduphone. 



