222 
PSYCHOLOGY: A. C. HARDY 
Proc. N. a. S. 
Results of this sort should be of interest, not only to the illumination 
engineer, but to the physiologist and the psychologist as well. If the 
number of observers were large to insure that the results represent the 
average eye, it would be possible to construct a map of the retina with 
"contour lines" to show equal values of the persistence of vision. This 
was done by the author using the values obtained for his own eyes. The 
general shape of the lines was found to coincide more or . less with the shape 
of the color fields given by Abney.^ The extent of the color fields is, 
of course, dependent upon the intensity of the light. It was not possible 
to show that the area of the retina covered in this investigation was 
greater than the color field for the blue for the intensity used. As the 
color field for the blue is larger in area than for any other color, it seems 
natural to suppose that the persistence of vision should depend only upon 
the intensity of the light on portions of the retina outside this area and 
should be independent of the wave-length. 
Description of apparatus. — The persistence of vision was measured by 
observing the minimum speed at which a sectored disk could be driven 
without destroying the apparent continuity of the light. The source of 
light was a concentrated filament incandescent lamp operated at constant 
voltage. A lens system was used to bring the rays to focus on the sec- 
tored disk. When the position of the disk is such that the rays do not 
strike it, they diverge until they strike a ground-glass screen about 5 
centimeters square. An iris diaphragm placed just in front of it makes 
the size of the illuminated area on the ground-glass adjustable without 
altering the brightness. The sectored disk, the necessary electric motor 
to drive it, the incandescent lamp and the lens system are all placed in a 
light tight box. The eye was then placed 1 meter in front of the ground- 
glass and a chin rest was provided to insure steady conditions of the retina 
while making the observations. Needless to say, the investigation was 
carried on in total darkness. A small electric lamp operated on the 
storage battery current and carefully shielded was used to read the instru- 
ments when necessary. The time for the recovery of the retina after 
this stimulus was less than the time required to place the apparatus in 
adjustment for the next reading. 
The speed of the disk was measured by means of a small magneto and 
a voltmeter calibrated to read the speed directly in revolutions per minute. 
The persistence of vision was first determined for the fovea by causing 
the disk to rotate at sufficient speed so that no flicker was apparent and 
then slowly to lose velocity until the first flicker was observed. On the 
average, it was found possible to determine the critical speed so that subse- 
quent readings would not differ by more than 2 per cent. Observa- 
tions were also made with the speed of the disk increasing and the average 
was taken as the persistence measure. A set of filters made by the Wratten 
