696 SPECIAL SENSES 



stationary, as the period of time necessary for 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 illustrate these points. In a disk marked with 

 alternate radiating lines of -black and white, the rays become indistin- 

 guishable 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 pro- 

 duced in this way, the resultant color appearing 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, 0.21 of a second. 



The impressions that 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 powerful and prolonged, 

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

 after-images. 



It is unnecessary to describe further in detail the well-known phe- 

 nomena 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 scien- 

 tific toys producing optical 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 sometimes propagated to those immediately adjacent. This gives 

 to objects a certain degree of amplification, which usually is in propor- 

 tion 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 on a black ground. Although the di- 

 mensions of the two circles are identical, 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 portion 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 on which the rays of light directly impinge, but that the sensitive 

 elements immediately contiguous are always more or less involved. In 

 looking at powerfully-illuminated objects, the irradiation is considerable, 

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



In experiments analogous to those just described, made with strongly 



