572 Scattering of Light in Refractive Media of the Eye. 



the light diffracted by the pupil (supposed perfectly circular) 

 in a direction making 30' of arc with the source would be 

 only 3 u00 00Q of that seen in the direction of the source. (This 

 is calculated from the formula Ia[J 1 (z)/z~\ 2 , the radius of the 

 pupil being taken to be 2 mm. and X=5600 A.U.) Actually , 

 the streamers surrounding the source can be seen in directions 

 making an angle of 200' and even more with its direction. 

 and it seems safe to say that their intensity half a degree 

 away from the source is a much greater fraction of its 

 apparent intensity than — ^- 000 . 



A further test of the view that the effects are principally 

 due to the structure of the eye and not to diffraction at the 

 margin of the pupil, is furnished by experiments with very 

 thin metal screens containing apertures placed in front of 

 the pupil of the eye. Using a circular hole with smooth 

 edges smaller than the pupil and placed in front of it, the 

 intensity of the streamers surrounding the source of light is 

 reduced, but does not vanish. When the screen is turned 

 about an axis normal to its plane, the hole being continually 

 kept in front of the pupil, the streamers of light seen in the 

 field remain visible and fixed in position, showing that they 

 are due to the structures of the eye through which the light 

 passes, and not to the margin of the pupil, or the edges of 

 the hole. Another and probably more convincing demon- 

 stration is obtained by using a square aperture smaller than 

 the pupil of the eye and placed in front of it. (This may 

 easily be contrived with the aid of four Gillette blades forming 

 the four sides of a very small aperture.) In this case, the 

 effect due to diffraction at the boundaries of the aperture is 

 very clear and marked, but can be shown (on rotating the 

 aperture in its own plane) to be entirely distinct and separate 

 from the phenomena now under discussion. 



The angular diameters and intensities of the haloes are such 

 as to suggest that we are dealing not with one but with two 

 sets of structures contained in the refractive media of the 

 eye, averaging in size about 13 yu, and 7 /jl respectively. The 

 structures of the latter (smaller) size are indicated by the 

 outermost halo, which appears to be composite in character 

 and due to the superposed effect of the two sets of particles. 

 These structures in the living eye are presumably to be 

 localised in the cornea and in the vitreous humour, as histo- 

 logical evidence of the existence of cellular structures in 

 these bodies is available. Upon this question, however, the 

 author does not venture to express any opinion. 

 Calcutta, 

 April 22nd, 1919. 



