76 Scientific Proceedings, Royal Dublin Society. 
pencil-point be now brought in the field of view on the stage 
and looked at, accommodation takes place, and the pupillary dise 
will be seen to contract; the pencil may be suddenly raised near 
the eye-piece, when a further contraction occurs, owing to the 
convergence of the eyes that now takes place. 
Other causes which give rise to pupillary contraction or dilata- 
tion, such as an emotional disturbance, fright, pain, and the action 
of certain drugs, &c., can be studied more readily, and their effect 
more accurately measured by the Entoptiscope than in any other 
way. A magnified view can also be obtained of a slight rhythmic 
contraction of the pupil (said to be seen by some, though I have 
not noticed it), which appears to be connected with respiration 
and the systole of the heart. 
§ 8. | 
(4). One of the most striking phenomena observed with the 
Entoptiscope is the extraordinary displacement of the projected 
image of the pupil which takes place when it is seen first by direct 
and then by oblique vision. This change in the position of the 
image, due to a change in the point of view, is not the ordinary 
parallactic displacement; for this does not occur in the Entoptiscope, 
but is, I find, an ocular parallaw due to the structure of the eye. 
The position of an object when seen by direct vision becomes 
apparently displaced when it is seen by indirect vision, that is 
when the pencil of rays from the object is oblique to the visual 
axis. This displacement increases with the obliquity of the rays 
falling on the pupil, and therefore with the angular magnitude 
of the cone of rays entering the eye. Hence, if the object were at 
an infinite distance, it would vanish; hence also the larger the 
area of the pupil the greater the ocular parallax; if the pupil 
were a point, it would disappear. I must reserve to a subsequent 
paper the explanation and discussion of this obscure subject, 
together with the series of measurements I have made in connexion 
with it. It will be sufficient here to describe a few experiments 
on this parallax which can be made with the Entoptiscope.” 
1In this I have been aided by my assistant, Mr. Warwick, A.R.C.Sc., to whom 
I am also indebted for several of the drawings in this and the previous paper. 
2 A paper ‘‘On the Law of visible position in single and binocular vision,”’ 
