Hacxett—The Photometry of N-Rays. 133 
are most suitable for these observations. I shall, therefore, mention a few points 
which seem worthy of special emphasis. 
An object whose image is not larger than the area of the fovea is occasionally 
the only one suitable for observation, but it needs great care in its use, as one 
needs to keep the eyes consciously fixed on it. It is extremely easy to imagine 
that the gaze is fixed directly on such an object; whereas its image is displaced 
over the retina, and consequently appears brighter than when on the fovea. 
Lummer has already emphasized this point, but I was surprised at the ease 
of self-deception on this matter. In fact, it was the greatest difficulty in getting 
the curves of equi-sensitiveness, as it was necessary that one should keep conscious 
control over the eyes. 
A larger object is less trying to observe, but it gives the eye greater freedom 
of motion, causing much apparent variation. The means adopted to avoid this 
may seem artificial, but they correspond to the peculiar character of the dark- 
adapted retina. In order to observe small changes when the screens were 
bright, the device of slits was used. This device makes the variation caused by 
eye-movements quite plain. But after some practice the appearance of the object 
can be kept constant. After many trials a diaphragm, easy to observe and 
sensitive to small variations in brightness, which enabled the observer to control 
these motions of the eye, was adopted. The particular dimensions of such a 
diaphragm would depend on the individual. The general form used resembled a 
capital H, with an additional vertical line down the middle. This gives three 
slits crossed by a horizontal band, which served as a spot on which to keep the 
eye fixed. In one case the length was about 5 em., the broad horizontal band 
I cm. deep, and the slits were 8 mm. wide, separated by an interval of 4 mm. 
This diaphragm was used with screens which had the brightness acquired in 
diffused daylight. The distance of observations of screen and diaphragm was 
about 80 cm. in Mr. Quinlan’s case, and about 50 em. in my own. At, these 
respective distances the slits became blurred, and slight changes in brightness 
caused them to re-appear again. 
The object was kept of constant appearance by keeping the eyes directed 
towards the central band, and correcting any changes which might occur by 
consciously re-directing the eye towards it. Any change which could not be so 
corrected cannot be a change due to any displacement on the retina. In special 
cases this correction may be assisted by the peculiar change in appearance caused 
by such displacements. 
It is necessary to mention some other subjective causes which may disturb the 
appearance of the object when its image on the retina is large. The screen 
apparently increases in brightness (1) by resting that part of the retina on which 
