Light in the, Refractive Media of the Eye. 569 



An enormous number of luminous streamers appear to emerge 

 from the source, and stretch out from it in more or less 

 exactly radial directions. Helmholtz was of opinion that 

 these streamers were due to the diffraction of light at the 

 irregular margin of the pupil of the eye; and this view is 

 supported in his treatise by a description of the phenomena 

 observed when the source of light is seen through a small 

 hole in a metal plate placed in front of the eye so as 

 partially or wholly to screen the margin of the pupil. Some 

 doubt as to the adequacy of the explanation advanced by 

 Helmholtz having been felt by me, a careful study of the 

 effects was undertaken, the results of which are described in 

 the present paper. The phenomena being of a subjective 

 character, the assistance of a number of independent ob- 

 servers with normal vision was obtained in order to confirm 

 my personal observations. This appeared all the more 

 necessary, as the conclusions arrived at as to the origin of 

 the phenomena differ from those of Helmholtz. 



2. Description of the Phenomena. 



The character of the luminous phenomena seen is widely 

 different in the two cases in which the source emits white 

 and monochromatic light respectively. For observing the 

 phenomena with white light, the most suitable arrangement 

 is to condense the light of an electric arc upon a pin-hole in a 

 large dark screen, and to view the issuing light with the eye 

 placed at a distance of about three or four yards from the 

 screen. The luminous pin-hole appears surrounded in the 

 first instance by a circular patch full of luminous streaks 

 starting out more or less radially from it, and occasionally 

 crossing each other. These streamers are generally white, 

 but appear here and there tipped with streaks of colour. The 

 circular patch is surrounded by a relatively dark ring, 

 outside which again the streamers reappear passing radially 

 through a luminous coloured halo * surrounding the dark 

 ring. The halo is, in fact, made up of short sections of the 

 streamers which, here, are strongly coloured. Outside the 

 halo, the streamers emerge again, but are much fainter, and 

 they form a broad and somewhat ill-defined ring of: lumi- 

 nosity extending to a considerable angular width from the 

 source. The inner margin of this luminous ring is greenish- 

 blue, and the outermost visible periphery is of an orange-red 

 colour, but some fainter fluctuations of luminosity and colour 

 may be observed within it. 



For observations with monochromatic light, a Westinghouse 



* Of angular radius a little less than '2 degrees of are. 



