In concluding this discussion of the Weber's law experiment I wish to call attention to the chief facts which have been revealed, and to make a critical comment. In my opinion it is extremely important that the student of animal behavior should note the fact that the dancer with which I worked week after week in the Weber's law investigation gradually improved in her ability to discriminate on the basis of brightness differences until she was able to distinguish from one another two boxes whose difference in illumination was less than one tenth[1] that of the brighter box. At the beginning of the experiments a difference of one half did not enable her to choose as certainly as did a difference of one tenth after she had chosen several hundred times. Evidently we are prone to underestimate the educability of our animal subjects. [Footnote 1: Under the conditions of the experiment I was unable to distinguish the electric-boxes when they differed by less than one twentieth.] The reason that the experiments were carried out with only one mouse must now be apparent. It was a matter of time. The reader must not suppose that my study of this subject is completed. It is merely well begun, and I report it here in its unfinished state for the sake of the value of the method which I have worked out, rather than for the purpose of presenting the definite results which I obtained with No. 51. The critical comment which I wish to make for the benefit of those who are working on similar problems is this. The phosphor bronze wires, on the bottom of the electric-boxes, by means of which an electric shock could be given to the mouse when it chose the wrong box, are needless sources of variability in the illumination of the boxes. They reflect the light into the eyes of the mouse too strongly, and unless they are kept perfectly clean and bright, serious inequalities of illumination appear. To avoid these undesirable conditions I propose hereafter to use a box within a box, so that the wires shall be hidden from the view of the animal as it attempts to discriminate.