Page:Proceedings of the Royal Society of London Vol 60.djvu/404

Effect of Pressure on Temperature of Crater Arc. strong illumination therefore the sympathetic dark reaction would seem to be least in the case of the red fibres.

Subjective colours of the same class as those shown by Benham’s top, but not nearly so conspicuous, have long been known. Helmholtz* mentions that if a rotating disk with black and white sectors is looked at fixedly, each white sector appears to be reddish along its leading border and bluish along its rear border. He also remarks that these colours are more easily seen upon a disk covered with two spiral bands, black and white, of equal breadth. From these and other observations, Helmholtz concludes that when a point of the retina is exposed to rapid alternations of white light and of darkness, causing successive states of increasing and decreasing excitation, the moment of maximum excitation is not the same for all colours. It has, however, been shown above that in analogous cases the red originates in a portion of the retina which has not been exposed to the direct action of light, while the blue originates in a portion where light has not ceased to act. Helmholtz’s supposition therefore does not apply—at least to the class of colours at present under consideration.

I have not made any attempt to account for the more feeble colours exhibited by the two intermediate groups of lines in Benham’s top, nor for the changes which occur when the speed of rotation is increased. These effects no doubt result, at least in part, from modifications of the phenomena already discussed. But for the present I am compelled to discontinue the experiments on account of the disagreeable and probably injurious effects which they produce upon the eyes.

In May, 1895, a preliminary paper by one of the authors was read at the Royal Society, in which is described the apparatus used for these experiments, and the results which were then obtained. The primary object of this research was to determine, if possible, whether the temperature of the crater in the positive carbon varies when the pressure in the surrounding gas is changed.

It has been suggested that the temperature of the crater is that of


 * ‘ Phys. Optik,’ § 23.