74 Scientific Proceedings, Royal Dublin Society. 
will be noticed to be brighter than the other portions of the 
duplicated pupillary disc.’ 
The observer will probably notice that on first looking through 
the Entoptiscope the two circles of light barely overlap; but, as 
the pupil dilates in the subdued light, the overlapping rapidly 
increases, soon reaches a maximum, and becomes steady, if the 
illumination remains unchanged. ‘This is due to the fact that, as 
the pupil dilates, its projected image enlarges, and the double 
images or discs of light therefore encroach on each other. Hence, 
before making any measurements, a moment or two should elapse 
to allow for the dilatation of the pupil. The duplicating of the 
image of the pupil isa very delicate method of measuring changes 
in its magnitude. 
x ! 
Hh NiO. 
Si. 
As the Entoptiscope is designed chiefly for the use of the 
ophthalmic surgeon, it would be the presumption of ignorance on 
my part if I entered into its clinical use. But there are several 
interesting phenomena connected with vision which, I venture to 
think, will render the instrument of value in physiological and 
psycho-physical demonstration and research. To some of these 
I will now briefly allude. 
1 This drawing of the obscurities in my left eye I made nearly six months later 
than that shown in fig. 1, Plate LV. ; the slow growth of the cataract is thus seen ; the 
principal addition is the smaller bar parallel to and below the large pen-like horizontal 
obscurity: the field is darker, and a few additional spots are also scattered about. 
