88 Scientific Proceedings, Royal Dublin Society. 
or micro-photographs of objects magnified 7000 diameters exhibit 
a wonderful clearness and sharpness of definition, as was shown in 
Mr. Gordon’s lecture on the subject at the Royal Institution.' 
(8). The so-called Haidinger’s brushes may be mentioned in 
concluding the summary of these entoptic phenomena. These are 
seen by some when the sky, or any brightly illuminated white 
surface, is viewed through a Nicol’s prism. A pair of faint 
yellow tufts or sectors, shaped something like an hour-glass, 
is seen in the plane of polarization; and, at right angles, the 
space is filled with a faint blue light. These coloured sectors 
rotate as the Nicol is turned, showing that the eye can act 
as an analyser to polarized light. Jamin, Brewster, Helmholtz, 
and others have suggested various explanations of this phenomenon, 
which probably depends on a slightly polarizing structure possessed 
by the cornea or the fovea centralis of the eye.” 
I cannot conclude this paper without adding my humble 
tribute of admiration to the amazing genius and almost miracu- 
lous range of knowledge possessed by that great Englishman, 
Dr. Thomas Young, the extent and value of whose discoveries in 
vision are not even now adequately recognised, though, from the 
first, Continental physicists and ophthalmologists have done him 
greater honour than his own countrymen. 
1 Vide Journal of the Royal Microscopical Society, 1903, pp. 400 e¢ seg; also 
Proceedings of the Royal Institution, February 17, 1905. 
Vide Brewster’s Optics, pp. 245 et seg.; Helmholtz, Optique Physiologique, pp. 552 
et seg.; G. G. Stokes, Brit. Assoc. Report, 1850,p. 20. Drawings of Haidinger’s 
tufts and the exact measurement of the retinal area they cover, as seen by the 
author and by one of his senior students, Mr. Ledwidge, will be published shortly, 
as some light is thus thrown on the seat of this obscure ocular phenomenon. 
