182 Hackxert— The Photometry of N-Rays. 
50 to 60 cm. in this experiment. But as I am a little short-sighted, my range 
was usually from 30 to 40 cm. with the same object. This difference in distance 
of observation has no bearing on the results, because curves got by Mr. Quinlan at 
30 em. did not differ from those got at 50 cm. in their general features. But it is 
evidently more advantageous to get readings at the greatest distance which suits 
the observer. 
These curves, besides affording some definite information about the observer’s 
eye, are also interesting from a physiological point of view. The contrast 
between the inner and outer curves in fig. 1 is striking. It is intimately 
connected with the physiology of the eye. The yellow spot is, as is well known, 
an oval with its long axis horizontal. This axis is usually about 2 mm. long, and 
the short axis is stated to be about two-thirds of this. It will be noted that the 
innermost oval closely corresponds to these dimensions. In fact, this curve was © 
very often obtained in the investigations. It would seem, then, the boundary of 
the yellow spot is the transition curve from circles to ovals. I have got no inside 
curves in my own case, owing to the causes mentioned above, but observations 
in Mr. Quinlan’s eyes confirm this fact. 
Again, physiologists ascribe the increased sensitiveness to the recovery of the 
visual purple in the dark. The distribution of visual purple increases outwards 
from the fovea centralis, and there is none in the fovea centralis. If this be the 
case, then these curves indicate how the density of visual purple increases. As 
long as we are inside the yellow spot, the change of distribution is symmetrical 
with respect to the circular boundary of the fovea; while outside, the distribution 
is symmetrical with respect to the oval boundary of the yellow spot itself. I am 
not aware that the retina has ever been examined from this point of view. 
It should be mentioned, in connection with this experiment, that resting the 
eyes or any ordinary stimulation of the retina fails to cause any change in the 
appearance of the slits as long as their image falls on the fovea. The question 
how fatigue or stimulation affects the other parts of the retina is too complicated 
and difficult to be investigated in the manner described. 
The curves indicate the extent of the change in the appearance of an object 
by the displacement of its image on the retina. To an inexperienced person, a 
phosphorescent screen appears to be going through a series of bewildering 
variations. The greater part of these, if not all, are due to motions of the eye. 
They gradually diminish, as practice gives greater control over the eye. 
The principal facts to be noted are that the most common motions of the eye 
are horizontal ; but, as a set-off, a vertical movement in general causes a greater 
change than an equal horizontal movement. It isan important question, then, for 
any observer to know how to get these motions under control, and what objects 
