A B C D E less = gr. less = gr. less = gr. less = gr. less = gr. R. (a) 2 0 23 0 0 25 4 4 17 3 1 21 0 1 24 (b) 3 1 21 1 0 24 5 3 17 1 6 18 0 2 23 F. (a) 1 4 20 3 0 22 8 6 11 0 5 20 2 0 23 (b) 2 3 20 2 1 22 6 7 12 1 4 20 0 3 22 K. (a) 4 2 19 4 0 21 2 7 16 0 7 18 0 0 25 (b) 1 0 24 2 6 17 8 0 17 2 6 17 1 0 24 -------- -------- -------- -------- -------- 13 10 127 12 7 131 33 27 90 7 29 114 3 6 141 TABLES IV.-VIII. The first line in column _A_ (Table IV.) signifies that out of 10 judgments, comparing an open space 4 cm., total weight 20 gr., with a filled space of 4 points, total weight also 20 gr., the filled space was judged less 7 times, equal 2 times, and greater once. III. The results of the investigation, thus far, point to the conclusion that short filled spaces are underestimated, that long spaces are overestimated, and that between the two there lies what might be called an 'indifference zone.' This unexpected outcome explains, I think, the divergent opinions of the earlier investigators of this problem. Each theory is right in what it affirms, but wrong in what it implicitly or openly denies. 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.