ILLUSIONS OF VISION 469 



the beauty of the phenomena repays the effort. There is a sharpness 

 and a density about the inner halo around the spot itself which does 

 not characterize the ordinary outer halo. For such differences I have 

 no explanation to offer. 



Not only is the cause of these details very difficult of detection, 

 but the origin of the whole halo phenomenon is equally so. It prob- 

 ably lies in the obscure reactions that change light waves into nerve 

 impulses. One thing which the intensified zone does do is to help 

 correct for rays which the irregular refraction of the eye scatters across 

 a margin ; and so this light area fulfils some psychological necessity. 



The fact that in the first flash of after-images this zone becomes 

 occupied by the color of the object looked at (like the common positive 

 after-image) suggests that it is a zone in a condition of expectant 

 attention with reference to that color. If, for example, a red disk 

 is observed, the nerves that per- 

 ceive that color are in full ac- 

 tivity, where the stimulus of the 

 image f^lls on the retina. For 

 a certain distance away from the 

 active retina, they are aroused into 

 a condition of readiness for ac- 

 tivity or expectant attention. The 

 secondary image acts like the fa- 

 tigue area, for it reverses in the 

 after-image. 



The siofnificance and applica- _ D 



& J- J- Fig. 7. Stellar Rays. 



tion of the phenomena are easier. 



From the psychological standpoint, its immediate application is to 

 questions of contrast. Contrasts are divided into two classes: First, 

 successive contrast, due to fatigue and rest ; second, simultaneous or 

 marginal contrast, now seen to be a subordinate part of this halo 

 phenomena. Marginal contrast has been long known, and its after- 

 image, the ' Lichthof ' of Hering, has been described. The fact that the 

 halo phenomenon definitely limits the region of marginal contrast and 

 displays a secondary image in a definite position proves it to be the 

 more fundamental phenomenon. We have here, therefore, a new illu- 

 sion of interest to psychologists and of great significance in its appli- 

 cation to astronomical work. 



Eats 



Unlike the halo, the ray phenomena are familiar and involve no 

 new principle, but the idea of rays around a black spot is new to me, 

 and quite as important as the halo in its application to visual work 

 by telescope or microscope. As all know, the rays on a star are pro- 



