DUKATION OF LUMINOUS IMPRESSIONS. 127 



image of a peculiar lustre, even when both surfaces are dull. 

 This may be very elegantly shown by making a stereoscopic 

 combination of images of crystals, one with black lines on a 

 white ground, and the other with white lines on a black 

 ground. The resulting image has then the appearance of 

 dark, brilliant crystals, like graphite. 1 



Duration of Luminous Impressions. 



The time necessary for vision is exceedingly short ; so 

 short, indeed, that it almost passes our powers of comprehen- 

 sion. Taking advantage of the very delicate methods of chro- 

 nometric observations now employed by physicists, it has been 

 shown by Prof. Rood that the letters on a printed page are 

 distinctly seen when illuminated by an electric spark, the du- 

 ration of which was measured and found to be not more 

 than forty billionths of a second. 2 By reference to page 74, 

 it will be seen that the waves of light strike the eye at the 

 rate of over five hundred millions of millions in a second ; so 

 that, even in the period indicated by Prof. Rood, an immense 

 number of waves have time to impinge upon the retina. 



We have long been familiar with the fact that an impres- 

 sion made upon the retina endures for a period of time that 

 can readily be measured, and that its duration bears a certain 

 degree of relation to the intensity of the luminous excitation. 

 If, after looking fixedly at a very bright object, we suddenly 

 produce complete obscurity, the object is more or less dis- 

 tinctly seen, when the rays have ceased to pass to the eye, 

 and the image fades away gradually. When we produce a 

 rapid succession of images, they may be, as it were, fused 

 into one, as the spokes of a rapidly-revolving wheel are in- 

 distinct and produce a single impression. This is due to the 

 persistence of the successive retinal impressions ; for, if a re- 

 volving wheel, or even a falling body, be illuminated for the 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 983. 



2 ROOD, On the Amount of Time necessary for Vision. American Journal of 

 Science and Arts, New Haven, 1871, Third Series, vol. ii., p. 159. 



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