measured on two subjects, is the statement that that person will see more ‘rods’ whose after-image persists longer.  This result the present writer fully confirms.  What relation the ‘constant product’ bears to the duration of after-image will be spoken of later.  But aside from all measurement, a little consideration of the conditions obtaining when the rod is passed behind the disc will convince any observer that the bands are indeed after-images somehow dependent on the rod.  We may account it established that the bands are after-images.

From this beginning one might have expected to find in the paper of Jastrow and Moorehouse a complete explanation of the illusion.  On other points, however, these authors are less explicit.  The changes in width of the bands corresponding to different sizes of the sectors and different rates of movement for the rod and disc, are not explained, nor yet, what is more important, the color-phenomena.  In particular the fact needs to be explained, that the moving rod analyzes the apparently homogeneous color of the disc; or, as Jastrow and Moorehouse state it (op. cit., p. 202):  “If two rotating discs were presented to us, the one pure white in color, and the other of ideally perfect spectral colors in proper proportion, so as to give a precisely similar white, we could not distinguish between the two; but by simply passing a rod in front of them and observing in the one case but not in the other the parallel rows of colored bands, we could at once pronounce the former to be composite, and the latter simple.  In the indefinitely brief moment during which the rod interrupts the vision of the disc, the eye obtains an impression sufficient to analyze to some extent into its elements this rapid mixture of stimuli.”  The very question is as to how the eye obtains the ‘impression sufficient to analyze’ the mixture.

It may be shown at this point that the mistake of these authors lies in their recognition of but one set of bands, namely (ibid., p. 201), ‘bands of a color similar to that present in greater proportion’ on the disc.  But, on the other hand, it is to be emphasized that those bands are separated from one another, not by the fused color of the disc, as one should infer from the article, but by other bands, which are, for their part, of a color similar to that present in lesser proportion.  Thus, bands of the two colors alternate; and either color of band is with equal ease to be distinguished from the fused color of the main portion of the disc.

Why our authors make this mistake is also clear.  They first studied the illusion with the smaller sector of the disc open, and the rod moving behind it; and since in this case the bands are separated by strips not of the minority but of the fused color, and are of about the width of the rod itself, these authors came to recognize bands of but one sort, and to call these ‘images of the rod.’  But now,