50 Scientific Proceedings, Royal Dublin Society. 
focus f’, and the shadow # will, accordingly, be larger than 0; in 
the latter the rays will converge, and would meet, if prolonged, 
in the conjugate focus f’ ; accordingly, the shadow (3 is somewhat 
smaller than the object 6. Hence, in estimating the actual magni- 
tude of any obscurity within the eye it is desirable to place the 
pin-hole diaphragm at the position of the anterior focus, a little 
over half an inch (14 mm.) from the surface of the cornea. 
Not only the actual size of a microscopic object within the eye can 
be thus found, but, as will be shown presently, its exact distance 
from the retina can be determined. 
In order to delineate an entoptic object, and estimate its magni- 
tude, the method hitherto employed is one of considerable difficulty 
to an untrained observer. It is what is known as the method of 
double vision, and was first suggested by Donders. The procedure 
is familiar to microscopists; one eye views an object through 
hig, 4b 
a microscope, whilst the other eye looks at a sheet of paper on 
which the projected image of the microscopic object is seen, and 
can be sketched with a little practice. Placing an object of known 
dimensions, such as a finely divided scale, under the microscope, 
the linear magnifying power of the microscope is found by com- 
paring the actual size of a scale-division with the width apart 
of the pencil-marks (seen with the other eye), which are placed 
