color,’ one in which the second color is in the background and is viewed through an opening in the first.  With such an arrangement we find that we get the series of bands both when the wire is passed in front of the disc and when passed in back between disc and background; and further experimentation shows that the time relations of the two are the same. (There is, of course, no essential difference between the two methods when the wire is passed in front of the disc.)” That is true, but it is to be borne in mind that there is a difference when the wire is passed behind the disc, as these authors themselves state (loc. cit., note):—­“The time-relations in the two cases are the same, but the color-phenomena considerably different.”  However, “these facts enable us to formulate our first generalization, viz., that for all purposes here relevant [i.e., to a study of the time-relations] the seeing of a wire now against one background and then immediately against another is the same as its now appearing and then disappearing; a rapid succession of changed appearances is equivalent to a rapid alternation of appearance and disappearance.  Why this is so we are unable to say,” etc.  These authors now take the first step toward explaining the illusion.  In their words (op. cit., p. 205), “the suggestion is natural that we are dealing with the phenomena of after-images....  If this is the true explanation of the fact that several rods are seen, then we should, with different rotation rates of disc and rod, see as many rods as multiplied by the time of one rotation of the disc would yield a constant, i.e., the time of an after image of the kind under consideration.”  For two subjects, J.J. and G.M., the following tabulation was made.

         &nb
sp;                                               J.J.  G.M. 
Av. time of rot. of disc when 2 images of rod were seen .0812 sec. .0750 sec.
  " " " " 3 " " " " .0571 " .0505 "
  " " " " 4 " " " " .0450 " .0357 "
  " " " " 5 " " " " .0350 " .0293 "
  " " " " 6 " " " " .0302 " .0262 "

“Multiplying the number of rods by the rotation rate we get for J.J. an average time of after image of .1740 sec. (a little over 1/6 sec.) with an average deviation of .0057 (3.2%); for G.M. .1492 (a little over 1/7 sec.) with an average deviation of .0036 (2.6%).  An independent test of the time of after-image of J.J. and G.M. by observing when a black dot on a rotating white disc just failed to form a ring resulted in showing in every instance a longer time for the former than for the latter.”  That this constant product of the number of ‘rods’ seen by the time of one rotation of the disc equals the duration of after-image of the rod is established, then, only by inference.  More indubitable, since directly