An Optical Phenomenon

Abstract

IN addition to the accounts in NATURE referred to by Mr. J. W. Giltay in your issue of November 22, the phenomenon mentioned by me in NATURE of October 18 had previously been far more fully described by Dr. John Aitken in a paper “On a New Variety of Ocular Spectrum” in the Journal of Anatomy and Physiology, vol. xiii., p. 322; and, as stated in this paper, the phenomenon noticed by Mr. C. Carus Wilson (NATURE, October 25), when travelling by train in a rear coupe compartment, was described by Prof. Silvanus Thompson in the report of the British Association for 1877. Dr. Aitken experimented with rotating discs divided into about twenty-four sectors, white and black alternately, and with endless bands of paper with black bars painted across them. A convenient way of viewing the apparent motion was to look at a sheet of mottled paper, after looking at the rotating disc or moving band, when the markings on the paper appeared to move in a contrary direction to the exciting impression. Though some of the mottlings seem to flow past the others, it was found by Dr. Aitken that a straight line drawn across “the spectral stream” did not appear to be bent, as one might have supposed would be the case. If, after viewing the rotating disc, another similar disc or the drawing of a wheel is looked at, the second disc or the wheel appears to rotate in a contrary direction to the first; but if the second disc is larger than the first, or the spokes of the wheel are extended to a greater size than the rotating disc, “this extension will entirely destroy all appearance of rotation, and the wheel will appear at rest. Do not these last experiments suggest that the seat of illusion is deeper than the retina?”