Beginning with cases of _total_ blindness (including under this head those upon whom light has simply a general subjective "heat-effect," enabling them to distinguish between night and day, between shade and sunshine, but inducing little or no tendency to project the cause of the sensation into the external world), I find on my list fifty-eight such cases. Of these, thirty-two became blind before the completion of their fifth year, and _not one_ of this group of thirty-two sees in dreams. Six became blind between the fifth and the seventh year: of these, four have dreams of seeing, but two of them do so seldom and with some vagueness; while two never dream of seeing at all. Of twenty persons who became blind after their seventh year _all_ have "dream-vision"--as I shall term the faculty of seeing in dreams. _The period from the fifth to the seventh year is thus indicated as the critical one._ Before this age the visual centre is undergoing its elementary education; its life is closely dependent upon the constant food-supply of sensations; and when these are cut off by blindness, it degenerates and decays. If blindness occurs between the fifth and the seventh years, the preservation of the visualizing power depends on the degree of development of the individual. If the faculty is retained, it is neither stable nor pronounced. If sight is lost after the seventh year, the sight-centre can, in spite of the loss, maintain its function; and the dreams of such an individual may be hardly distinguishable from those of a seeing person. It was a very unexpected discovery, to find, after I had planned and partly completed this investigation, that I had a predecessor. So long ago as 1838, Dr. G. Heermann studied the dreams of the blind with the view of determining this same question, the physiological significance of which, however, was not then clearly understood. He records the answers of fourteen totally blind persons who lost their sight previous to their fifth year, and _none_ of these has dream-vision. Of four who lost their sight between the fifth and the seventh year, one has dream-vision; one has it dimly and occasionally; and two do not definitely know. Of thirty-five who became blind after their seventh year _all_ have dream-vision. The two independent researches thus yield the very same conclusion. Dr. Heermann includes in his list many aged persons, and from their answers is able to conclude that, generally speaking, those who become blind in mature life retain the power of dream-vision longer than those who become blind nearer the critical age of five to seven years. He records twelve cases where dream-vision still continues after a blindness of from ten to fifteen years, four of from fifteen to twenty years, four of from twenty to twenty-five years, and one of thirty-five years. In one case dream-vision was maintained for fifty-two, and in another for fifty-four years, but then faded out.[15]