490 JAMES CLERK MAXWELL. 



being so cut out of the sheet and put together that its fibrous 

 structure, due to rolling in the process of manufacture, should 

 radiate, at least approximately, from a central point. This arrange- 

 ment, as I understood at the time, in polarised light, reproduced, or 

 rather simulated, Haidinger's phenomenon. But after an interval 

 of thirty years I must speak with caution. Brewster (British 

 Association Report, 1850), ascribed Haidinger's phenomenon 

 to " polarising structure existing in the cornea and crystalline 

 lens, as well as in the tissues which lie in front of the sensitive 

 layer of the retina ; " while Stokes proved that, when the variously 

 coloured rays of the prismatic spectrum were admitted separately 

 into the eye, in the blue rays alone could Haidinger's Brushes be 

 seen. A few years later, in his paper " On the Unequal Sensi- 

 bility of the Foramen Centrale to Light of Different Colours " 

 (Brit. Assoc. Report, 1856), Clerk Maxwell says that, on looking 

 through a prism at a long vertical slit he saw an elongated dark 

 spot running up and down the spectrum, but refusing to pass out 

 of the blue into the other colours. This appearance, he concludes, 

 is due to the " Foramen centrale " of Soemmering ; and he adds 

 that when a Nicol's prism is employed the brushes of Haidinger 

 are well seen in connection ivith the spot. The appearance of a 

 dark spot on a blue ground only, Maxwell then, or at least 

 afterwards, believed to be due to the yellow pigment of the 

 macula lutea, of which the fovea centralis, as its name imports, 

 is the middle portion, and, as is well known, the place of most 

 distinct vision, having a special selective absorption for the blue 

 rays. His notable discovery that Haidinger's Brushes were only 

 to be seen in connection with the shadow of the yellow spot 

 thus pointed conclusively to the spot itself as the seat of the 

 phenomenon ; or, as he himself puts it, makes evident the fact 

 that the brushes are the spot analysed by polarised light. 

 Having thus, to his own satisfaction, localised the polarising 

 structure concerned in the production of Haidinger's phenomenon, 

 he seems to have rested, for I am not aware that he ever wrote 

 again on the subject. No such writing at least appears in the 

 Royal Society catalogue of scientific memoirs. Possibly he hoped 

 that some day he might himself examine the actual structure of 

 the yellow spot ; or he may have waited for the result of such 

 examination by other hands than his own. Helmholtz, who cites 

 Maxwell's paper of 1856, attributes the phenomenon of the 

 brushes to a radiating fibrous structure, which, it seems, has 

 actually now been ascertained to exist in the fovea centralis, 

 which he assumes to be feebly polarising, and to possess a special 

 selective absorbing power for the blue rays. (Helmholtz, 



