Barrerr—On Entoptic Vision. 7d 
The observer, having adjusted the inclination of the in- 
strument to suit his convenience, moves the mirror until a 
brilliant and uniform light covers the ground-glass stage. A 
shaded lamp, or incandescent gas-mantle, or a couple of incan- 
descent electric lights, or the bright light of the sky, can be 
used as the source of light. It is, however, preferable to employ 
a constant source of light, such as the incandescent mantle 
placed at a given distance from the mirror. Both eyes should 
be kept open, and buried up to their sockets in the eye-cups, the 
head-rest being moved until the forehead is comfortably supported. 
(1). When the observer’s eyes are adjusted in the eye-cups, 
and the brightly lighted stage seen through the smallest apertures 
in each eye-piece, the graduated sector, as already stated, at once 
indicates the exact distance between the pupils, if this is required. 
(2). One of the revolving diaphragms is then turned to O, to 
occlude whichever eye is not under observation. Hven without a 
transparent scale in the eye-piece or on the stage, the observer 
can readily note the well-known fact that light falling on one 
eye causes doth pupils to contract simultaneously. ‘This ‘ con- 
sensual reaction,’ as it is termed, and its rate, can be accurately 
studied with the Entoptiscope. It is only necessary to open 
suddenly the largest aperture in front of one eye, keeping the 
smallest aperture before the other, when the contraction of both 
pupils will be seen. Still better, if the stage be dimmed by 
altering the mirror, an electric lamp brought close under one eye, 
with the largest aperture open, though not seen by the other 
eye, nevertheless causes a vigorous contraction of its pupil. The 
scale, or the ruled squares on the tracing-paper, enables the con- 
traction to be measured, whilst the periodic time of the consensual 
reaction can be determined by opening and closing the large 
aperture in front of the electric lamp. 
(3). The change of the pupil, during accommodation, has 
already been mentioned. This ‘accommodative contraction,’ as 
well as the direct light reaction in the pupil, is strikingly seen in 
the Entoptiscope. Turn both diaphragms to O; then, after a few 
moments in darkness, open an aperture in one eye-piece. The 
contraction of the pupil from light-reaction is first seen. If a 
