Barretr—On Entoptic Vision. 69 
assume the same law holds true when a stenopic screen! (7.¢., a pin- 
hole diaphragm) is used, and the entoptic shadow is seen projected 
through the pin-hole aperture on to a surface beyond. But, as I 
have explained in Part I., § 8, this is not the case, and could not 
be the case, for the reasons I there adduced. Careful measurements 
which I have made show, as might be expected, that the magnifi- 
cation is in the ratio of the distance d of the pin-hole from the 
pupil, to the distance D of the pin-hole from the ground-glass 
stage or other surface on which the image is projected, as shown 
in fig. 6. Hence, if the shadow on the retina be the same size as 
the object (as occurs when the pin-hole is at the anterior focus of 
the eye), the linear magnitude z, of any obscurity within the eye, 
is known when that of the projected image S is measured, for 
d 
waa 8. 
As the pupil is approximately 3 mm. from the cornea, this 
amount must be added to the distance d of the orifice from the eye. 
§ 4. 
But the use of the Entoptiscope is not confined to the detection 
and delineation of opacities in the eyeball. The circular image 
the observer notices on the stage is not an enlarged view of the 
orifice through which he is gazing, but, as already mentioned, it is 
a magnified image of his own pupil. The iris limits the divergent 
cone of rays entering the eye, and its shadow is sharply depicted 
on the retina. Hence any irregularities in the iris are at once 
detected, and can be readily traced by the patient, and the exact 
size of the pupil accurately determined. Pupillometry, as it is 
called, is one branch of ophthalmology ; and the ease, expedition, 
and accuracy with which it can be accomplished by means of the 
Entoptiscope will, I hope, render this instrument, in the hands of 
oculists, a useful means of supplementing the usual external 
methods of observation of the pupil. Even among the limited 
number who have so far used the Entoptiscope, I have been 
1 As ‘stenopaic’ screen (see Part I., p. 44, footnote) may mean either a narrow 
slit or a small pin-hole in a screen, I shall use the word ‘stendpic’ to signify the 
latter only. 
