When I was in college I was very fond of handball and was a member of the handball team. It has been many years since I played the game, yet I can distinctly feel the peculiar tension of the right arm and shoulder muscles that accompanied the "service." Nor do I feel the slightest difficulty in evoking a distinct mental image of the prickly sensations that so annoyed me as a boy when I would first put on woolen underwear in the fall of the year. [Sidenote: _Personal Differences in Mental Imagery_] From these examples, it is apparent that we can form mental images of past sensations of sight, sound, taste, smell and feeling, and indeed of every kind, including the muscular or motor sense and the sense of heat and cold. But there is the greatest possible difference in individuals in this respect. Some persons have distinct images of things they have seen, are good visualizers. Others are weak in this respect, but have clear auditory images. And so as to all the various kinds of sensory images. This is a fact of comparatively recent discovery. The first proponent of the idea was Fechner, but no statistical work was done in this line until Galton entered the field, in 1880. In his "Inquiries into Human Faculties," he says: [Sidenote: _Investigations of Doctor Galton_] "To my astonishment, I found that the great majority of the men of science to whom I first applied protested that mental imagery was unknown to them, and they looked on me as fanciful and fantastic in supposing that the words 'mental imagery' really expressed what I believed everybody supposed them to mean. They had no more notion of its true nature than a color-blind man, who has not discerned his defect, has of the nature of color. They had a mental deficiency of which they were unaware and naturally enough supposed that those who affirmed they possessed it were romancing." [Sidenote: _Investigations of Professor James_] The investigations of Dr. Galton were continued by Professor James, of Harvard University. He collected from hundreds of persons descriptions of their own mental images. The following are extracts from two cases of distinctly different types. The one who is a good visualizer says: