Johannes Götz

“ Light is now cut off, but heat passes through freely. The focus is absolutely dark, but it still contains heat, of which fact we can soon convince ourselves by introducing a cigar into it: it is ignited and bursts into flame. ”

“ We now turn to the opposite end of the spectrum—the ultra-red rays. Our proposition is, "Are we able to render perceptible to the eye, the organ of sight, those rays that operate upon our sense of feeling simply as conveyers of heat?" We can attain our purpose only by augmenting the number of vibrations the thermal rays, by their influence upon suitable bodies, in such a manner that they are rendered perceptible to the visual organ. We provide the electric lamp with a parabolic reflector, A B, silvered and polished within, with incandescent carbon-points in its focus. ”

“ It begins at once to shine vividly with a blue light. A writing with cyanuret of barium and platinum upon white paper is invisible in ordinary white light, but, as soon as we expose it in the ultra-violet end of the spectrum, it emits greenish-blue light. Finally, if we throw the spectrum upon a screen, the paper covering of which is saturated with quinine sulphate, we shall at once observe that it extends largely beyond the violet end. The ultra-violet rays now begin to appear with a pale-blue color. ”

“ There are those known, the light of which disappears very quickly after its emission, and again those by which the illumination lasts as long as eighteen hours—of course, while growing constantly feebler. ”

“ A later experiment will answer the question.
There is an occurrence very generally found in nature which is dissimilar in form, but analogous in essence, with the fluorescence. With fluorescent substances, the emission of light ceases as soon as illumination is interrupted. If light is thrown upon calcium preparations, a part of the rays is also absorbed, and altered into rays of another number of waves. But these preparations emit the absorbed rays partly only after the cessation of illumination. ”

“ White as well as black paper is charred and ignites in it. A piece of platinum-foil is raised to white heat in the dark focus. If we examine the incandescent platinum with a prism, we find that it emits again all colors, from the most extreme red to the most extreme violet; consequently, we have here the counterpart of fluorescence. The dark rays, by the augmentation of the number of vibrations, are converted into luminous ones, influencing the eye. ”

“ Various inferior organisms are provided with special glands for secreting phosphorescent substance. During excitement this is exuded by the animal, and begins to emit light. The phosphorescent light of the tropical waters is produced by myriads of minute organisms, by which the substance is secreted. ”