Page:Proceedings of the Royal Society of London Vol 60.djvu/401

374 equal parts by a straight line from the centre to the circumference, and one of these parts was painted black. The disk was attached to a horizontal spindle, turned by a motor at the rate of five or six revolutions per second, while its front was illuminated by a lamp of 16-candle power. A white card, upon which was a black line, or a design composed of black lines, was supported behind the disk, and viewed intermittently through the open sector. When the rotation was such that the open sector succeeded the black portion of the disk and was succeeded by the white portion, the black lines became red. This experiment is identical with the last, except that the white ground is illuminated entirely by reflected light. In conjunction with the others, it indicates with certainty the origin of the remarkable red colour shown by Benham’s top.

The disk with the open sector affords a much more convenient means than the top of exhibiting the colour phenomena. If a disk with an open sectpr of 45° or 60° is made of white cardboard, and a movable black half disk is mounted in front of it upon the same axis, we may, by suitably adjusting the position of the black half disk with regard to the opening, produce in a fixed object all the tints shown by the top, as well as intermediate ones; and the object itself may be easily changed to suit the conditions of an experiment.

If the commutator of Experiment V, or the disk with the open sector of Experiment YI, be turned in the reverse direction, the strips of tinfoil or the black lines appear to become blue (instead of red), like the outer group of lines in Benham’s top when it spins in the direction indicated by the arrow in the figure. This appearance is partly, if not altogether, illusory. It is the bright ground in the immediate neighbourhood of the black lines that becomes blue; the lines themselves (except possibly just within their extreme edges) become a neutral grey, owing to the alternations of light and darkness or of white and black.

A card with some black lines 1 mm. thick drawn upon it was placed behind the disk with the open sector of Experiment YI, which was turned in the direction such that the' open sector was preceded by white and followed by black. The lines presented the appearance of having been drawn with blue ink upon imperfectly sized paper, a blue stain having apparently spread for a short distance on both sides of the lines.

Lines of gradually increased thickness were successively employed until at last they had the form of bands -f-in. wide ; and even in this latter case it was not easy to see that the bands themselves did not become blue, but only their outlying borders.