I should have been willing some years ago to name without hesitation a third condition of discrimination--saying it would be most developed in that organ which is susceptible of the _most various qualities_ of feeling. The retina is unquestionably such an organ. The colors and shades it perceives are infinitely more numerous than the diversities of skin-sensation. And it can feel at once white and black, whilst the ear can in nowise so feel sound and silence. But the late researches of Donaldson, Blix, and Goldscheider,[170] on specific points for heat, cold, pressure, and pain in the skin; the older ones of Czermak (repeated later by Klug in Ludwig's laboratory), showing that a hot and a cold compass-point are no more easily discriminated as two than two of equal temperature; and some unpublished experiments of my own--all disincline me to make much of this condition now.[171] There is, however, one quality of sensation which is particularly exciting, and that is the _feeling of motion over any of our surfaces_. The erection of this into a separate elementary quality of sensibility is one of the most recent of psychological achievements, and is worthy of detaining us a while at this point. _The Sensation of Motion over Surfaces._ _The feeling of motion_ has generally been assumed by physiologists to be impossible until the positions of _terminus a quo_ and _terminus ad quem_ are severally cognized, and the successive occupancies of these positions by the moving body are perceived to be separated by a distinct interval of time.[172] As a matter of fact, however, we cognize only the very slowest motions in this way. Seeing the hand of a clock at XII and afterwards at VI, we judge that it has moved through the interval. Seeing the sun now in the east and again in the west, I infer it to have passed over my head. But we can only _infer_ that which we already generically know in some more direct fashion, and it is experimentally certain that we have the feeling of motion given us as a direct and simple _sensation_. Czermak long ago pointed out the difference between seeing the motion of the second-hand of a watch, when we look directly at it, and noticing the fact of its having altered its position when we fix our gaze upon some other point of the dial-plate. In the first case we have a specific quality of sensation which is absent in the second. If the reader will find a portion of his skin--the arm, for example--where a pair of compass-points an inch apart are felt as one impression, and if he will then trace lines a tenth of an inch long on that spot with a pencil-point, he will be distinctly aware of the point's motion and vaguely aware of the direction of the motion. The perception of the motion here is certainly not derived from a pre-existing knowledge that its starting and ending points are separate positions in space, because positions in space ten times wider apart fail to be discriminated as such when excited by the dividers.