70 Scientific Proceedings, Royal Dublin Society. 
struck with the deformation of the pupil occurring and drawn 
in one or two cases, though the defect was not conspicuous 
externally to any casual observer. 
It is also interesting to note the wide variation in the size 
of the pupil among different people, under the same degree of 
illumination and with similar accommodative contraction. The 
average normal pupil is rather less than 4 mm. in diameter; and 
a difference in diameter of a fraction of a millimetre is easily 
registered by the Entoptiscope. For the purpose of measurement 
it is better not to attempt to trace the whole circle of light seen 
on the stage, but simply to make a pencil-mark at the extremities 
of any diameter of the circle, and measure the width apart of the 
pencil-marks by a pair of compasses. A transparent millimetre- 
scale can of course be placed on the stage, or the eye-piece 
micrometer-scale can be used, and the diameter of the circle read 
off directly ; but for an inexperienced observer the pencil-marks 
I find more satisfactory. The diameter of the pupil being ob- 
tained, if it be a normal or fairly circular pupil, it is a great help 
to draw on the ground glass, or tracing-paper, a circle of this 
diameter, with the cross-lines of fig. 5 as the centre of each circle, 
using for this purpose a compass or a coin of the right size. This 
enables the patient to make his drawing of any entoptic objects 
with more ease and leisure; still better is it to use as a guide an 
opaque diaphragm, with a circular aperture of the right size, laid 
on the stage. Ora movable opaque screen can be used, having 
two apertures—one corresponding to each eye, and each about 
2 cm. diameter. As the magnification of the pupillary image 
depends on the distance of the screen from the pin-hole, by 
making this screen with a sliding-tube, to enable it to move up or 
down the pillar of the instrument whilst it is kept parallel to the 
stage below, a position will be found by the observer when the 
diameter of the aperture in the screen exactly corresponds with 
the area of the image of his pupil; the screen is then kept in this 
position whilst the drawing is made below. 
§ 5. 
I have already mentioned in Part I. that it was the i 
of a small permanent obscurity in both of my eyes, before any 
