Vol. 6, 1920 
PSYCHOLOGY: A. C HARDY 
223 
and Wainwright Company was used one at a time when it was desired to 
use light of a particular color. These filters were found to be very nearly 
monochromatic. The use of spectrum colors would be more accurate 
but the intensity of the light cannot be adjusted within as wide limits. 
To determine the persistence of vision for off center portions of the 
retina, a small radiolight sight was used. This was mounted on a slider 
attached to a long rod and so constructed as to revolve about the center of 
the diaphragm. In this way it was possible to place the sight in any 
desired position with respect to the center of the diaphragm. Shallow 
grooves were placed at intervals along the rod so that it was possible to 
read the position of the slider in the dark. In the experimental work, 
readings were taken about every 3 degrees from the center and along 
directions which made angles with the horizontal of 45, 90, 135, 180, 225, 
270 and 315 degrees. The manipulation was the same as before except 
that the attention was directed toward the radiolight sight and the per- 
sistence of vision measured with the light from the ground-glass screen 
falling on some other portion of the retina. It was, of course, necessary 
to cover one eye during all of the experimental work. 
Experimental results. — Before results could be obtained which were 
consistent with themselves, it was found necessary to take several pre- 
cautions. For example, time was given for the eye to become accustomed 
to the darkness. Results were obtained which showed that 5 minutes in 
total darkness was sufficient. It was also found that any motion of the 
body, however slight, would cause the interest to flag. For this reason, 
the motor controls had to be adjusted so that the motor would change its 
speed slowly as it was impossible to operate a rheostat by hand. One 
hand was held on a key which was pressed at the instant that the flicker 
was seen to appear or disappear and the critical speed noted. 
The size of the diaphragm which seemed to give the best results was a 
circle of diameter 5.84 mm. The persistence of vision is dependent upon 
the size of the retinal area stimulated and also the scintillation of the light 
from a small aperture caused more or less uncertainty.^ The above aper- 
ture was chosen as being the smallest that it was practicable to use. With 
the diaphragm placed at a distance of 1 meter from the eye, the angle 
subtended by the diaphragm at the eye is 3.36°. 
As has already been said, the persistence of vision was determined for 
several colors and in each case the persistence was measured for about 
one hundred points on the retina lying inside a circle which is the base of 
a cone whose semivertical angle is 38.7°. No attempt will be made to 
give the results in full. They represent the persistence of an impression 
on the retina of the eye of the author. The eye is known to be normal for 
color perception but has a moderate amount of astigmatism which should 
not affect the persistence of vision. A few results will be given to show the 
nature of the inferences which may be drawn from the investigation. 
