ILLUSIONS OF VISION 47 1 



Irradiation. — Next to the black rays in importance is the matter 

 of irradiation as analyzed by means of ray forms. The method of 

 investigation is as follows: Make a small hole in a window blind and 

 observe the sky through it by different distances. From near-by 

 the outline of the hole is well perceived, but, as one draws away, the 

 rays soon obscure the hole itself, so that its form and size can not be 

 perceived. At these different distances, the width of the rays varies 

 with the true angular size of the hole. For example, I found at ten 

 feet an irradiation of 2' and at three feet an irradiation of 6', because 

 at the nearer point the rays are three times as wide and overlap each 

 other at three times the distance from the hole. Irradiation then is 

 the merging together of the rays, and on any straight line of separa- 

 tion, is proportional to the total ray light on the corresponding hemi- 

 sphere about a star. 



Color, size, intensity and perfection of eye are positive factors in 

 irradiation. The negative factor is the background, and the result 

 depends upon the sensitiveness of the eye at the time of observation. 

 Some general results we can see at once. Irradiation is not neces- 

 sarily the same in any two eyes or in any two directions. It varies 

 with fatigue of the retina and probably with use of the eye in some 

 unusual position, producing unusual conditions of pressure upon the 

 eye-ball. Ordinarily, its amount depends directly on contrast between 

 the areas observed and on the size of the central nucleus of rays in 

 the desired direction. This nucleus must not be assumed to be cen- 

 trally located on its source. 



Best Part of Lens. — A very important bit of information derived 

 in this study of the rays is the location and size of the best part of 

 the crystalline lens. This is done by trying smaller and smaller dia- 

 phragms over the eye until the rays cease to appear. They will be 

 found to persist in rudimentary form even when the diaphragm is 

 as small as 1/16 of an inch. This is of great significance in tele- 

 scopic and microscopic work, because it shows how small the emergent 

 pencil of light must be to avoid the excessive formation of rays. Even 

 at best, they can not be hindered entirely. The use of lower powers 

 with large emergent pencil is therefore very dangerous. The optically 

 superior part of the lens occupies a small irregular area near the center 

 with irregular extensions out toward the margin. Even the best part 

 is far from perfect. 



Detached Spots. — An interesting variation of stellar rays has been 

 observed at least in one case. A gentleman drew for me the rays as 

 they appeared to his eye in the experiment described above (see Fig. 

 7), and while working asked me if I had placed a number of smaller 

 pin holes around the large one. Fig. 10 represents this. It is per- 

 fectly possible for detached spots of this kind to be produced by some 



