DURATION OF LUMINOUS IMPRESSIONS 695 



eyes ; for the estimation of distance is obtained by bringing the axes of 

 both eyes to bear on a single object, be it near or remote. The fact is 

 as was distinctly stated by Galen in the second century that in look- 

 ing at any solid object not so far removed as to render the visual axes 

 practically parallel, a portion of the surface seen with the right eye is 

 not seen with the left eye, and vice versa. The two impressions, there- 

 fore, are not identical for each retina ; the image on the left retina includ- 

 ing a portion of the left side of the object, not seen with the right eye, the 

 right image in the same way including a portion of the right surface, not 

 seen with the left eye. These slightly dissimilar impressions are fused and 

 produce the impression of a single image, when vision is normal; and 

 this gives the idea of relief or solidity and an exact appreciation of the 

 form of objects, when they are not too remote. 



Although an opposite opinion is held by some experimenters, Helm- 

 holtz, with many others, has stated that when one color is seen with one 

 eye and another color, with the other eye, in the stereoscope, the im- 

 pression 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 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 combination observed in ordinary experi- 

 ments. One additional point of importance, however, is that the binoc- 

 ular 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 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. 



Duration of Luminous Impressions {After-Images). The time re- 

 quired for a single visual stimulation of the retina is short. The letters 

 on a printed page are distinctly seen when illuminated by an electric 

 spark, the duration of which is not more than forty-billionths of a second 

 (Rood). An impression made on the retina, however, endures for a 

 length of time that bears a certain relation to the intensity of the lumi- 

 nous excitation. If the eyes are closed after looking steadily at a bright 

 object, 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 

 there is a rapid succession of images, they may be 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 is 

 illuminated for the brief duration of an electric spark, it appears to be 



