114 Scientific Proceedings, Royal Dublin Society. 
In from three to seven seconds the tufts disappear if the plane 
of polarization of the Nicol be kept stationary, the period of their 
duration becoming shorter, and their shape less distinct, as the 
retina grows fatigued from observation. When a blue glass is. 
held before the Nicol, the tufts appear somewhat smaller, and of a 
dark-reddish tint, due to the absorption of the yellow rays by the 
cobalt-blue glass, which transmits the extreme red as well as the 
blue. Though smaller, the dark tufts seen through a blue glass 
are somewhat more conspicuous than the yellow when viewed on 
a bright sky. Hence, if any observer finds a difficulty in seeing 
the yellow sectors, he should try a blue glass, rotating the Nicol 
through 90° every two or three seconds. 
The fugitive appearance of the tufts when the conditions of 
observation are unchanged is characteristic of several other entoptic 
phenomena. Thus the macula lutea, or yellow spot, which can 
easily be seen by the use of proper absorbent media (see p. 120), soon 
disappears when looked at steadily. It is, however, less fugitive 
than Haidinger’s tufts; on the other hand, the tufts or polarized 
sectors can be kept continually in view by a very slow rotation 
of the Nicol’s prism: even one revolution in half a minute I find 
enables them to be kept in sight, their longer axis revolving at 
the same rate as the Nicol. 
Fig. 2, Plate VI., shows the appearance, size, and tint of the 
yellow tufts, as seen projected on a brightly illuminated sheet 
of white paper placed 150 to 160 cm. (about 5 feet) from the 
eye.| When the plane of polarization is horizontal, and the 
Nicol suddenly removed from the eye, the tufts will still be 
seen faintly, but in a plane at right angles to their previous 
position. This is, no doubt, due to the feeble polarization of 
the light reflected from the paper, the Nicol in the first instance 
localizing their position, for if black glass or a polished surface 
be substituted for the paper, the effect is much more conspicuous ; 
moreover, under such circumstances, the tufts can be seen in a 
vertical plane without using the Nicol at all. The light of the 
sky is polarized at 90° from the sun; hence those who are 
familiar with the appearance of the yellow tufts can faintly 
see them without the aid of a Nicol by looking at a bright sky 
1 The plane of polarization in this case is horizontal. 
