I next set out to determine as precisely as possible how far the factor of fusion, or what Parrish has called irradiation, enters into the judgments.  It was evident from the beginning of this whole investigation that fusion or displacement of the points was very common.  The term ‘irradiation’ is, however, too specific a term to describe a process that works in these two opposite directions.  The primary concern of these next experiments was, therefore, to devise means for preventing fusion among the points before the subject pronounced his judgment.  With our apparatus we were able to make a number of experiments that show, in an interesting way, the results that follow when the sensations are not permitted to fuse.  It is only the shorter distances that concern us here.  The longer distances have already been shown to follow the law of optical illusion, that is, that filled space is overestimated.  The object of the present experiments is to bring the shorter distances under the same law, by showing, first, that the objective conditions as they have existed in our experiments thus far are not parallel to those which we find in the optical illusion.  Second, that when the objective conditions are the same, the illusion for the shorter distances follows the law just stated.

In repeating some of the experiments reported in Tables IV.-VIII. with varying conditions, I first tried the plan of using metallic points at the ends of the spaces.  Thus, by an apparent difference in the temperature between the end points and the filling, the sensations from the end points, which play the most important part in the judgment of the length, were to a certain extent kept from fusing with the rest.  The figures in Table II. have already shown what may be expected when the points are kept from fusing.  Here, also, a marked tendency in the direction of apparent lengthening of the distance was at once observed.  These short filled distances, which had before been underestimated, were now overestimated.  The same results follow when metallic points are alternated with hard rubber points in the filling itself.

This changing of the apparent temperature of the end points has, it must be admitted, introduced another factor; and it might be objected that it was not so much the prevention of fusion as the change in the temperature that caused the judgments to drift towards overestimation.  I have statistics to show that this observation is in a way just.  Extremes in temperature, whether hot or cold, are interpreted as an increase in the amount of space.  This conclusion has also been reported from a number of other laboratories.  My contention at this point is simply that there are certain conditions under which these distances will be overestimated and that these are the very conditions which bring the phenomenon into closer correspondence with the optical illusion, both as to the stimuli and the subjective experience.  Then, aside from this, such an objection will be seen to be quite irrelevant