58 Scientific Proceedings, Royal Dublin Society. 
shadow R is the same size as the entoptic object, and hence the 
linear magnitude w of the pupil, or of the obscurity within the eye, 
is to that of its projected image S, as d is to D, or, 
d 
e=78. 
The area of # to that of S is of course as d? : D?. 
These measurements are easily made with the Hntoptiscope, 
nor does a slight deviation of the pin-hole from the anterior focus 
make any sensible error. 
S oe 
The distance of entoptic objects from the retina, as Listing has 
shown, can be ascertained by noting the relative parallactic dis- 
placement of the shadow which occurs upon moving the eye.’ 
This will be readily understood from an inspection of fig. 7, which 
is accurately drawn to scale. Within the eyeball let us suppose 
that three entoptic objects exist: No. 1 between the pupil and 
cornea; No. 2in the crystalline near the plane of the pupil; No. 3 
in the vitreous; therefore neater the retina than No. 2. The eye 
is first directed to the central spot of light or opening O in a 
stenopaic screen. The shadows of the three entoptic objects fall 
on the retina, and are seen as in the central projection. If now 
the eye be turned to view first the upper light or opening O’, and 
1 Listing, Beitrag zur physiologischen Optik. Gottingen, 1845. 
