Barrett—On Entoptic Vision. 63 
The first form of my Entoptiscope is shown in Plate III., fig. 1, 
and consists of a pair of vertical brass pillars supporting a head-rest, 
which can slide from side to side so as to bring either eye vertically 
over the pin-hole contained in the revolving diaphragm of the eye- 
piece. This diaphragm has pin-hole apertures varying in diameter 
from 0-1 to 2°5 of a millimetre, and a pair of pin-holes each 0'1 mm. 
diameter, and 2 mm. apart, so that by revolving the diaphragm 
either a single aperture of any given size or a double aperture 
can be successively brought before the eye. Figs. 1 and 2 show 
the diaphragm A in plan and elevation (the orifices are shown 
enlarged) ; the object of the double aperture will be seen later on ; 
an eyé-cup screws on to the upper part of the eye-piece (fig. 2). 
! 
jidigi 
Revolving 
Diaphragm 
Fig. 2. 
Below the pin-hole eye-piece is a transparent scale (fig. 3), 
divided into fractions of a millimetre; the shadow of this scale 
falls upon the eye of the observer, and is thence projected 
much magnified upon the ground-glass stage below, along with 
the shadows of any opacities seen in the eye. At the base of the 
instrument (Plate III., fig. 1) is a concave mirror, which can be 
adjusted so as to illuminate the eye-piece brilliantly, using the light 
of the sky or that of a lamp. A sharply-pointed and hard pencil 
L2 
