It is clear that if, instead of using as ‘rod’ a single radial sector, one were to rotate two or more such sectors disposed at equal angular intervals about the axis, one would have the same strobic phenomena, although they would be more complicated.  Indeed, a large number of rather narrow sectors can be used or, what is the same thing, a second disc with a row of holes at equal intervals about the circumference.  The disc used by the writer had a radius of 11 inches, and a concentric ring of 64 holes, each 3/8 of an inch in diameter, lying 10 inches from the center.  The observer looks through these holes at the color-disc behind.  The two discs need not be placed concentrically.

When produced in this way, the strobic illusion is exceedingly pretty.  Instead of straight, radial bands, one sees a number of brightly colored balls lying within a curving band of the other color and whirling backward or forward, or sometimes standing still.  Then these break up and another set forms, perhaps with the two colors changed about, and this then oscillates one way or the other.  A rainbow disc substituted for the disc of two sectors gives an indescribably complicated and brilliant effect; but the front disc must rotate more slowly.  This disc should in any case be geared for high speeds and should be turned by hand for the sake of variations in rate, and consequently in the strobic movement.

It has been seen that this stroboscope is not different in principle from the illusion of the resolution-bands which this paper has aimed to explain.  The resolution-bands depend wholly on the purely geometrical relations between the rod and the disc, whereby as both move the rod hides one sector after the other.  The only physiological principles involved are the familiar processes by which stimulations produce after-images, and by which the after-images of rapidly succeeding stimulations are summed, a certain number at a time, into a characteristic effect.

* * * * *

STUDIES IN MEMORY.

* * * * *

RECALL OF WORDS, OBJECTS AND MOVEMENTS.

BY HARVEY A. PETERSON.

Kirkpatrick,[1] in experimenting with 379 school children and college students, found that 3-1/3 times as many objects were recalled as visual words after an interval of three days.  The experiment consisted in showing successively 10 written names of common objects in the one case and 10 objects in the other at the rate of one every two seconds.  Three days later the persons were asked to recall as many of each series as possible, putting all of one series together.  The averages thus obtained were 1.89 words, 6.29 objects.  The children were not more dependent on the objects than the college students.

   [1] Kirkpatrick, E.A.:  PSYCHOLOGICAL, REVIEW, 1894, Vol.  I., p.
   602.