THE SMALLEST PERCEPTIBLE MOVEMENT.

Lake Wakatip Mail, Issue 2788, 11 October 1910, Page 2

THE SMALLEST PERCEPTIBLE MOVEMENT.

It is obvious that there must be a limit of magnitude below which movement is imperceptible to the human eye. In order to determine this limit some interesting experiments have been carried out by Dr. Adolf Basler. A narrow strip of white paper was attached to the short arm of a lever, and a millimeter scale was

placed behind the end of the long arm. The ratio between the arms being known, it was possible, by means of this arrangement, to move the strip of paper through a very small and accurately measured interval.

It was found that persons with normal acuteness of vision, looking at the strip of paper from a distance of 12 inches, could detect with certainty a movement of 1/800 inch, corresponding to a visual angle of about 20 seconds (owl/180th of a degree), and to a displacement of the image on the retina by 1/17000 inch. Several persons could detect n movement of the paper as small as 1/1200 inch. The smallness of these movements will be better appreciated when it. is recalled that two points 1/800 or 1/1200 inch apart cannot be distinguished as two points b\ the naked eye. Hence the eye can perceive a movement smaller than the interval between two points that appear as onr.

: This paradox may he explained as follows :—The sensitive layer of the retina (the back interior portion of the eyeball) is a mosaic of separate sensitive elements. Two points cannot he distinguished as two unless their images are separated by at least one element, which is not affected by either. On the other hand, the movement to a smaller extent of a luminous hody of considerable dimensions is perceived because the stimulus affects a number of retinal elements lying side by side. Therefore, in the movement of the edge of the image of the strip of paper the average displacement required is much less than the diameter of a retinal element.

In another confirmatory series of experiments, in which the distance between the strip of paper and the eye was increased to two yardß, the minimum perceptible movement wa< found to be proportionally increased, so that, the required annlar dis placement (20 seconds) and retinal displacement (1/17000 inch) remained the same as before.—" Science Sift' ings."