Barretr—On LEntoptic Vision. 113 
plane of polarization of the Nicol’s prism, and rotates with the 
rotation of the Nicol. 
If the Nicol prism be kept unmoved, the appearance fades 
away, and in a few seconds the tufts have vanished from view. 
When, however, the Nicol is suddenly turned to a different plane, 
they reappear, only to fade away again in five or six seconds, 
reappearing on every change in the plane of polarization. This 
fact, together with the faintness of the light of the tufts, explains 
why so many persons have failed to see them; for the longer the 
observer gazes through the Nicol, if the plane of polarization be 
unchanged, the more hopeless will be the search for ‘‘ Haidinger’s 
brushes.” Brewster states that, in spite of repeated efforts, it was 
more than twelve months before he saw the yellow tufts, or sectors}; 
and Helmholtz remarks that, ‘“ malgré les plus grands efforts,’ he 
tried in vain for twelve years before he perceived them.’ 
If the following instructions are attended to, few persons will 
have any difficulty in seeing what Haidinger discovered. Choose 
a bright sky, or a white surface brightly illuminated either with 
the sun, the electric light, or even an incandescent gas-mantle ; 
hold the Nicol close to one eye and shut the other eye; look through 
the Nicol at the illuminated surface, and there will be seen directly 
in the line of sight at the extremity of the optic axis, a faint 
yellow figure very like a little hour-glass, and occupying only a 
small part of the field of view, subtending an angle of about 3° to 
4° in fact. The general appearance and relative magnitude of the 
yellow tufts compared with the whole field of the Nicol’s prism 
when the open sky is viewed, are shown in Plate VI., fig. 1. The 
complementary violet tint which fills the space between the yellow 
tufts is more difficult to see, and has, by Jamin and some others, 
been attributed to the effect of colour contrast, and, therefore, 
to a purely subjective effect. This view is, however, rendered 
improbable from the fact that the blue tint is well seen when a 
polished black surface is used as the reflecting body. Moreover, 
one or two friends of mine have found that the blue sectors are 
seen more distinctly than the yellow, which latter is only faintly 
visible to one of these observers.* 
1 Brewster's Optics, p. 246. 
* Helmholtz, Optique Physiologique, p. 552. 3 See foot-note, p. 121. 
Q 2 
