RETINAL IMPRESSIONS. 465 



over the portions of the membrane contiguous to that directly exposed 

 to the action of the stimulus. This diffusion of luminous impressions 

 is known as irradiation. The phenomenon is particularly observed in 

 looking at bright objects on a darker ground. It is owing to irradia- 

 tion, that minute white objects painted on a black ground, not only 

 appear much larger, but are visible at greater distances than natural; 

 whilst, on the other hand, dark objects of the same size on a white 

 ground, appear smaller, and sooner become invisible at a distance. A 

 narrow bright strip of paper seems wider than a dark one of equal size. 

 The larger apparent size of the stars, as the sky becomes darker, and 

 the peculiar appearance noticed in the new moon, viz., that its light 

 crescent seems to belong to a larger sphere than the feebly illuminated 

 portion of its surface, are also due to irradiation. In certain cases, the 

 increased size, or blurred image, of a luminous object, is owing to de- 

 fective accommodation of the eye, and to dissipation of the luminous 

 rays. 



Impressions made upon the retina have a certain duration. They 

 continue to be perceived during a much longer interval than the im- 

 pressions which produced them, and their persistence is greater, the 

 greater the persistence of the original impressions. They are of the 

 nature of after-sensations, or the so-called spectra, and have been 

 named the primary or positive after-images. They last generally about 

 s^th of a second, but may endure for j a second. Their average 

 duration is about Jth of a second (Plateau). Were it not for the 

 duration of impressions, vision would not be continuous, for in every 

 act of winking, all surrounding objects would be lost sight of. Im- 

 pressions, occurring at shorter intervals of time than those just men- 

 tioned, are not perceived as distinct or separate. It is owing to the 

 duration of the sensation, that after looking at a vivid light or a bright 

 color, if the eyes be closed, or the head be suddenly turned away, 

 the impression continues for a certain period. The effects of different 

 colors presented, in rapid succession, to the eye, as illustrated in 

 experiments with the color-top, which is a spinning-top painted in dif- 

 ferently-colored segments, also depend on the duration of retinal im- 

 pressions. Various secondary or tertiary colors, and even a tolerably 

 pure white, are produced by the rapid rotation of differently- colored 

 tops. The appearance of a complete circle of light, which is seen on 

 rapidly whirling round a stick lighted at one end, the curved lines of 

 fire seen in a lighted " Catharine wheel," and the indistinct haze caused 

 by the rapid revolutions of the spokes or other parts of a wheel, are 

 explained in the same manner. If a small piece of cardboard, on one 

 side of which there is painted a bird, and on the other a cage, be made 

 to revolve rapidly by twisting strings fixed to each end, the bird ap- 

 pears to be in the cage. The toy known as a thaumatrope or strobo- 

 scope, consists of a disc, on which are painted sets of figures of men or 

 animals, in the different positions of some act, as, of leaping, running, 

 or tossing balls; when it is made to revolve, and the figures are looked 

 at through a slit, they are combined into one image, which appears to 

 be in motion. When two exactly similar toothed wheels, placed one 

 in front of the other, are made to revolve, in the same direction, and 



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