78 Scientific Proceedings, Royal Dublin Society. 
angular value of the parallax can be easily determined. It is 
this ocular parallax which gives rise to the mysterious and elusive 
dilatation of the pupillary disc that so puzzles the observer when, 
in using the Entoptiscope, he begins to trace the outline with a 
pencil. By keeping the eye always fixed on the pencil-point, and 
not allowing it to be beguiled by a furtive wandering of the axis of 
vision, a perfect tracing can be made. Diminishing the angle sub- 
tended by the projected image—which can be done by increasing the 
distance between the pin-hole and the eye—lessens the parallax. 
For in the Entoptiscope the greater the magnification, the greater 
the ocular parallax ; with distances of the pin-hole from the eye 
and to the stage respectively of 25 and 125 millimetres, or 1 to 5, 
an image is given of sufficient magnitude and having very little 
parallax. For this purpose the eye-cups are made detachable, 
and a second deeper pair supplied with the instrument. 
Another plan of avoiding this troublesome parallax in the 
Entoptiscope is to use a very small artificial pupil in the eye-piece, 
close to the eye; a disc with an aperture somewhat less than two 
millimetres in diameter almost stops the parallax; but this very 
much restricts the area to be seen either of the pupil or of the 
crystalline, and hence is not usually advisable. 
§ 9. 
(5). Upon winking the eyelids, transverse strive will be per- 
ceived on looking through the Entoptiscope, due probably to 
minute wrinkles in the epithelial layer of the cornea. If one eyelid 
be winked frequently, as happens in using a telescope or microscope 
for some time, strize are formed which remain for some hours, and 
give rise to a marked defect in vision. The presence of tears also 
gives rise to long strize from their prismatic action on the pencil of 
rays. Rubbing the cornea causes the roughening of the epithelial 
layer, which gives rise to a peculiar ribbed or mottled appearance 
in the field of view of the Entoptiscope, but this soon disappears. 
Dr. Thomas Young was, I find, the first to notice and depict this 
so long ago as 1801 (see footnote !, next page). Helmholtz gives 
some excellent drawings of these entoptic appeaianes ( Optique 
Physiologique, p. 208). 
