IERADIATION. 717 



tion. If the eyes be closed after looking steadily at a very 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 be illuminated for the brief duration of an electric spark, 

 it appears absolutely stationary, as the period of time necessary 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 experi- 

 ments made with revolving disks 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 produced by a combination of the black and white. 

 The effects of an artificial combination of colors may be produced in this way, 

 the resultant color appearing precisely as if the individual colors had been 

 ground together. 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, - 21 of a second. 



The impressions which remain on the retina after an object has been 

 looked at steadily are called after-images. When these are bright and of the 

 same character as the object, they are called positive after-images. When 

 the stimulation of the retina has been very powerful and prolonged, the 

 after-image frequently is dark. Such images are called negative after-images. 



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

 which illustrate the point under consideration. The circle 6f light produced 

 by rapidly revolving a burning coal, the track of a meteor, and other illustra- 

 tions, are sufficiently familiar, as well as many scientific toys producing opti- 

 cal illusions of various kinds. 



Irradiation. It has been observed that luminous impressions are not 

 always confined to the elements of the retina directly involved, but are some- 

 times 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 simple experiment of 

 looking at two circles, one black on a white ground, and the other white oh 

 a black ground. Although the actual dimensions of the two circles are iden- 

 tical, the irradiation of rays from the white circle makes this appear the 

 larger. In a circle with one half black and the other white, the white por- 

 tion will appear larger, for the same reason. These phenomena are due to 

 what has been called irradiation ; and their explanation is very simple. It is 

 probable that luminous impressions are never confined absolutely to those 

 parts of the retina upon which the rays of light directly impinge, but that 

 the sensitive elements immediately contiguous are always more or less in- 

 volved. In looking at powerfully illuminated objects, the irradiation is con- 

 siderable, as compared with objects which send fewer luminous rays to the eye. 

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