Page:On Radiation.djvu/35

 When this instrument is brought up to the violet end of the spectrum of the electric light, the heat is found to be insensible. As the pile gradually moves from the violet end towards the red, heat soon manifests itself, augmenting as we approach the red. Of all the colours of the visible spectrum the red possesses the highest heating power. On pushing the pile into the dark region beyond the red, the heat, instead of vanishing, rises suddenly and enormously in intensity, until at some distance beyond the red it attains a maximum. Moving the pile still forward, the thermal power falls, somewhat more suddenly than it rose. It then gradually shades away, but for a distance beyond the red greater than the length of the whole visible spectrum, signs of heat may be detected. Drawing, as Sir William Herschel did, a datum line, and erecting along it perpendiculars, proportional in length to the thermal intensity at the respective points, we obtain the extraordinary curve which exhibits the distribution of heat in the spectrum of the electric light. In the region of dark rays beyond the red the curve shoots up in a steep and massive peak—a kind of Matterhorn of heat, which dwarfs by its magnitude the portion of the diagram representing the luminous radiation. Indeed, the idea forced upon the mind by the inspection of this diagram is that the light rays are a mere