Barrerr—On Entoptic Vision. 115 
in a position at right angles to that of the sun. By means 
of Haidinger’s tufts the unaided eye can thus not only detect 
polarized light, but determine its plane of polarization." 
The tufts, however, cannot be seen, as was first pointed out by 
Sir G. Stokes,” in light of lower refrangibility than the green. 
Projecting a brilliant spectrum on to a white screen and looking 
at the colours through a Nicol’s prism, successively from the red. to 
the violet, it will be noticed that the tufts first appear in the green, 
about the F line, and that they remain visible until the limit of 
the more refrangible end of the visible spectrum is reached. ‘This 
explains why Haidinger’s tufts cannot be seen in artificial light 
of low refrangibility, though with ordinary gas-light they may 
be faintly perceived, not, however, yellow, but of a reddish tint. 
Various attempts have been made to explain this curious 
phenomenon, which for some years after its discovery excited much 
interest throughout the scientific world. Evidently the cause 
must be due to some structure in the eye which acts as an analyser 
to the polarized beam; but what that structure is has excited 
considerable discussion. We have also to consider the colour and 
shape of the tufts which suggest that a bi-refracting or depolariz- 
ing medium exists in the eye, the light transmitted through which 
being afterwards analysed by the polarizing structure referred to. 
J. T. Silbermann’ pointed out that both the cornea and 
crystalline lens of the human eye were in each case bi-refracting, 
but considered the lens to be the origin of the tufts, owing to its 
structure. This hypothesis was, however, shown to be incorrect 
by Zokalski,t an eminent Paris oculist, who found that the tufts 
could be seen when no crystalline lens was present in the eye, four 
aphakic patients of his seeing Haidinger’s tufts clearly. 
J. Jamin was of opinion that the cornea was the medium which 
1 Mr. C. T. Whitmell writes to me :—‘‘ Haidinger’s brushes have always interested 
me, for I see them every day when I glance at a polished table or at a shining black- 
board or book, or at the sky 90’ from the sun. Thus I can at once find, from the 
direction of the yellow brushes, the plane of polarization of the reflected light, without 
the aid of any apparatus but what everyone has in his own eye.” 
2 British Association Reports, 1850, Transactions of the Sections, p. 20. 
3 Comptes Rendus, 1846, vol. xxiii., p. 629. 
4 Quoted by Brewster, Comptes Rendus, 1859, vol. xlviii., p. 616. 
