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POPULAR SCIENCE MONTHLY 



may usually be seen complete. If the spots are larger, the halos can 

 not be traced within each other's precincts, and on enlarging the spots 

 still more, they soon act as one mark with regard to the halo, which 

 assumes an elliptical form around them. From these and other ex- 

 periments along the same line, it appears that the intensified zone or 

 white area, as I shall generally call it, referring to the negative experi- 

 ment, displays an increased sensitiveness to presence or absence of 

 color of the spot looked at, but a decided deadening in the perception 

 of details. 



My first idea in regard to this halo was that it came to life like 

 the camera ghost, from reflections between lens surfaces in the eye, 

 but I found that it could be produced through any portion of the 

 crystalline lens. A pin hole 1/50 inch in diameter passed before the 

 pupil of the eye demonstrated this. 



It then seemed possible that some form of halation in the mem- 

 branes close to the retina might produce this effect. The common 



photographic halation ring, which 

 closely resembles it, is produced by 

 reflection from the back of a glass 

 plate but can only occur under cer- 

 tain conditions. This halo, how- 

 ever, occurs on all margins and 

 can not be due to that cause. 



At this stage, a certain ' chro- 

 matic ring,' described below under 

 that heading, was observed, and 

 suggested some obscure color con- 

 ditions as the cause. Hence, color 

 tests were made in large numbers, 

 and the black spot was tried on dif- 

 ferent colored backgrounds with- 

 out effect. Different colored spots 

 against a dark background were 

 also observed without effect, save that the secondary image when suffi- 

 ciently bright was seen to be of the color of the spot itself; therefore 

 color was not responsible for the halo. 



But these color observations opened up a very interesting line of 

 study. The color tests had to be made in the positive form with all 

 the attendant difficulties of fatigue and after-images. It was found 

 that a short gaze at a red disk on a black background, followed by a 

 slight movement of the eye to one side, carried away a dark green 

 after-image of the disk surrounded by a red margin, about the size of 

 the intensified zone. This intensified zone became still more con- 



Fig. 2. Photographic Halation Ring 

 about Candle Flame, formed by reflection 

 inside the glass plate on which the picture 

 was taken, very similar in its appearance to 

 the halo here described. 



