ILLUSIONS OF VISION 465 



at the first view, it is a striking phenomenon. I find on the whole 

 that trained eyes are the ones which see it most quickly. 



A more beautiful and elegant way of making the experiment is by 

 standing a black-headed hat pin in the middle of a white-walled room, 

 and looking at it against the distant white background. Around the 

 head of the pin will then appear 

 this halo, more beautiful than 

 before, suspended in mid-air, in 

 the good old-fashioned manner of 

 saintly halos. 



The experiment described above _ 



gives the ' negative ' halo. It will 

 be generally referred to in this 

 article, because it is more easily 

 seen than the ' positive/ The 

 ( positive ' form of the halo, how- 

 ever, is most readily seen by a 



■ •!„_ 4.i,„i t i -L-4. Fig. 1. This Spot should be viewed 



similar method. Let a white- _. „ ^„ „ „„„ „ 



from a Distance of Six or Eight Feet, 



headed pin be Substituted for the with care to avoid fatigue or after-images, in 



■ i t i i i i • oider to see the fine dark halo ring about it 



Other, and looked at against a at the distance indicated by the smaller dot. 



black background. Similarly, a 



white circle is seen. The difficulties in this case arise from the 

 reflections on the head of the pin and its generally less even illu- 

 mination. 



The effect, however, is the same. Extending all round the head 

 of the pin at a distance of about 7' of arc (one inch at a distance of 

 500 inches) is an intensified zone in which the color of the background 

 appears stronger; and outside of that a reduction zone, or ring, or 

 secondary image, in which the intensity of the background is reduced 

 by the addition of some of the color of the spot observed. 



In order to find the cause of this halo, many tests were made, of 

 which the first was upon the size of the central spot, It was found 

 that the distance from the edge of the spot to the secondary image is 

 constant; that the width of the secondary image increases to some 

 extent with the size of the spot, and that the intensified area increases 

 its intensification with the size of the spot. If the spot is so small 

 as to be barely visible, the halo may still be seen, but the intensified 

 zone then appears of the same intensity as the background. 



If the spot is enlarged sufficiently, both positive and negative halos 

 are seen along its margin, one outside and one inside, so that in a 

 straight line separating light and dark areas, the positive halo may 

 be seen in the dark area, and the negative halo in the light. If two 

 small spots are placed so that their halos intersect, the halo of each 



VOL. lxx. — 30 



