806 SPECIAL SENSES. 



other eye, in the stereoscope, the impression is not of a single color resulting from the 

 combination of the two. It is true that there is an imperfect mingling of the two colors, 

 but this is very different from the resulting color produced by the actual fusion of the 

 two. There is, in other words, a sort of confusion of colors, without the complete com- 

 bination with which we are familiar in ordinary experiments. One additional point of 

 interest, however, is that the binocular fusion of two pictures, unequally illuminated or 

 of different colors, produces a single image of a peculiar lustre, even when both surfaces 

 are dull. This may be very strikingly shown by making a stereoscopic combination of 

 images of crystals, one witli 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 crys- 

 tals, like graphite. 



Duration of Luminous Impressions. 



The time necessary for vision is exceedingly short ; so short, indeed, that it almost 

 passes our powers of comprehension. Taking advantage of the very delicate methods of 

 chronometric observations now employed by physicists, it has been shown by Prof. Rood, 

 of New York, that the letters on a printed page are distinctly seen when illuminated by 

 an electric spark, the duration of which was measured and found to be not more than 

 forty billionths of a second. Inasmuch as the waves of light strike the eye at the rate of 

 over five hundred millions of millions in a second, it is evident that, even in the period 

 indicated by Prof. Eood, an immense number of waves have time to impinge upon the 

 retina. 



We have long been familiar with the fact that an impression made upon the retina 

 endures for a length 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 distinctly seen after 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 indistinct and 

 produce a single impression. This is due to the persistence of the successive retinal 

 impressions ; for, if a revolving wheel, or even a falling body, be illuminated for the brief 

 duration of an electric spark, it appears absolutely stationary, as the period of time neces- 

 sary for perfectly distinct vision and the duration of the illumination are so short, that 

 there is no time for any appreciable movement of the object. The familiar experiments 

 made with revolving disks strikingly illustrate these points. In a disk marked with 

 alternate radiating lines of black and white, the rays become entirely indistinguishable 

 during rapid revolution, and the disk appears of a uniform color, such as would be pro- 

 duced by a combination 6*f the black and white. Very beautiful effects of artificial com- 

 bination of colors may be produced in this way, the resultant color appearing precisely 

 as if the individual colors had been ground together. It is also interesting, in this con- 

 nection, to note that the duration of retinal impressions varies considerably for the 

 different colors. According to Emsmann, the duration for yellow is 0'25 of a second ; 

 for white, 0'25 of a second ; for red, 0-22 of a second ; and for blue, 0'21 of a second. 



It is unnecessary to describe farther in detail the well-known phenomena which 

 illustrate the point under consideration. The circle of light produced by rapidly revolving 

 a burning coal, the track of a meteor, and other illustrations, are sufficiently familiar, as 

 well as many scientific toys producing optical illusions of various kinds. 



Irradiation. It has been observed that luminous impressions are not always confined 



the elements of the retina directly involved, but are sometimes propagated to those 



immediately adjacent. This gives to objects a certain degree of amplification, which is 



generally in proportion to their brightness. An illustration of this is afforded by the 



imple experiment of looking at two circles, one black on a white ground, and the other 



