Light in the Refractive Media of the Eye. 571 



doubt that the phenomena described above have to be 

 referred to the diffraction of light by a large number of 

 particles of more or less uniform size included in the 

 structure of the refractive media of the eye. The peculiar 

 movements mentioned above would then naturally be as- 

 cribed to the movements of these particles. These may be 

 imitated by observing a source of light through a plate of 

 glass on which a little dilute milk has been flowed. The 

 movements of the diffracted streamers of light can then be 

 easily seen. 



Observations show that the angular diameters of the 

 circular patch containing the streamers and of the circular 

 haloes surrounding it are entirely independent of the aperture 

 of the pupil of ihe eye. This is readily proved by altering 

 the intensity of the source of light under observation, with 

 the result that the aperture of the pupil automatically 

 adjusts itself. An ordinary candle-flame at three metres 

 distance, and the light of an electric arc at the same distance 

 or even nearer the eye, give identical measurements for the 

 angular widths, though the aperture of the pupil must have 

 been greatly different in the two cases. This is exactly what 

 we might expect if the effects are due to particles contained 

 in the structure of the eye. but it is very difficult to reconcile 

 with the view of Helmholtz that the phenomena are due to 

 diffraction at the margin of the pupil. The observed effects 

 cannot be explained if we merely postulate any arbitrary 

 irregularities in the circular shape of the margin of the pupil. 

 It would be necessary also to assume a regular corrugation 

 or periodicity in the margin of the pupil*, and even such an 

 assumption, apart from its bein£ purely hypothetical, fails 

 to explain the observed effects. For, with any change in the 

 aperture of the pupil, the distance between successive 

 corrugations should also alter and influence the observed 

 phenomena. This is inconsistent with the observed inde- 

 pendence of the aperture and the observed effects. We are 

 thus led to reject the view that the diffraction at the margin 

 of the pupil determines the phenomena seen. 



It is useful in this connexion to note that the intensity of 



* A useful analogue is furnished by the milling on the circular edge 

 of a coin. In a paper on diffraction which is in course of publication. 

 S. K. Mitra has shown that a circular disk with corrugated edges pro- 

 duces a coloured halo surrounding the usual Fresnel-Arago central 

 brig;ht spot, and the angular radius of this halo is equal to >i\ --«, where 

 a is the radius of the disk and n is the number of corrugations, A 

 similar phenomenon is shown in convergent light by a circular aperture 

 with a corrugated edge, but in a less striking manner. The number o( 

 corrugations remaining the same, the angular radius of the halo would 

 vary inversely as the radius of the aperture. 



