Page:Proceedings of the Royal Society of London Vol 60.djvu/395

368 In addition to the arrangements above described for determining the capacity of the small condenser, we have also employed the wellknown method of charging and discharging the small condenser through a galvanometer by means of a contact-maker driven at a speed of sixty contacts per second by an electrically controlled tuning-fork. By this means a steady deflection of the galvanometer is obtained due to the passage of the rapidly recurring discharges through it. Preliminary observations with this apparatus have confirmed the above-given value for the dielectric constant of liquid oxygen, and by a modification of it we hope shortly to make a very careful re-determination of the constant.

The investigation which forms the subject of this paper originated in an attempt to account satisfactorily for the colour phenomena exhibited by Mr. C. E. Benham’s “Artificial Spectrum Top," which, when it was brought before the public, about two years ago, excited considerable interest.

The top consists of a disk of cardboard about 4£ in. (108 cm.) in diameter, mounted upon a spindle. One half of the disk is painted black; upon the white ground of the other half are drawn four successive groups of three black lines, having the form of concentric arcs of 45°, which are at different distances from the centre, as shown in the annexed figure; the thickness of the lines is about in. (1 mm.). When the disk rotates, each group of black lines generally appears to assume a different colour.

The nature of the colours thus developed depends upon the speed of the rotation, and upon the quality and intensity of the illumination. After several trials, I found that no better results, on the whole, could be obtained than when the disk was illuminated by a 16-candle power incandescent lamp, with a ground glass bulb, at a distance of about 6 in. (15 cm.), and was caused to turn about five times in a second. These, therefore, were adopted as the standard conditions for my experiments, the disk being mounted upon a horizontal axis, driven by an electro-motor, and the speed regulated by comparison with the ticks of an ordinary watch.

When the disk rotates under the specified conditions and in the direction indicated by the arrow in the figure, the inner group of