Barrett—On Entoptic Vision. 65 
reaching the eyes except through the single revolving diaphragm ; 
in the larger there are two revolving diaphragms, one (shown 
at D) for each of the eye-pieces HE (these are also shown at 
DD in fig. 4, p. 66). The eye that is not under observation 
is kept in complete darkness by turning D until the index 
marks O; at this position there is no aperture in the 
diaphragm ; thus either eye can be occluded with ease. The 
apertures are numbered in order of their size, No. 1 being less 
than a tenth of a millimetre in diameter, No. 2 larger, No. 3 
larger still, and soon. The mirror Mis plane and not concave, 
and made sufficiently large to cover the whole of the ground- 
glass stage G with a flood of light reflected from an adjoining 
incandescent gas lamp or other source of light. As the mirror 
is carried by the stage and moves with it, the illumination 
of the field remains unaltered in adjusting the inclination of the 
pillar to suit the observer. The pillar, stage, and mirror move 
with stiff friction round the centre A, and can be clamped 
in any position by turning a milled-head screw. In order to 
prevent any shifting of the observer’s head, and to avoid fatigue, 
a hinged and padded head-rest # is fixed in such a position that 
the forehead rests comfortably upon it. The head-rest is also 
made to rise or fall, and can be clamped at any elevation to suit 
the observer. 
§ 2, 
As the distance between the two eyes varies in different 
persons, the exact adjustment of the pupil to the pin-hole is 
accomplished by an ingenious device suggested to me by the 
remarkably skilful mechanician, Mr. M. Lambert, to whom I am 
indebted for the admirable construction of the instrument from the 
rough plans I gave him. This device is shown in fig. 4 (p. 66), 
and consists in gearing the arms supporting the eye-pieces, so that 
an automatic adjustment and exact centring occur directly the 
observer places his eyes in position. Incidentally this arrange- 
ment, by means of the scale on the sector S, affords a most perfect 
method of ascertaining the exact pupillary distance of the two eyes, 
thus replacing the spectacle “ trial-frame,” or sliding callipers, 
usually employed for this purpose by the oculist. 
