Geissler's tube, the exterior envelope of which contains four phosphorescent substances in four divisions (Fig. 2). By means of the electric current, we raise the temperature of the inclosed air to a glow-heat, and cause the emitted rays of light to operate for about a minute upon calcium salts.
After interrupting the light, the salts appear in four different colors, to wit: orange, yellow, green, and blue. This property of phosphorescence is universally found in nature. I call to mind the glowing of decayed wood, that of fire-bugs, etc. 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.
An Englishman, Balmain, succeeded some time ago in manufacturing a substance of a fairly intense and durable phosphorescence. It is used for painting watch-dials, match-boxes, door-signs, etc., to make them self-illuminating. In tenor with the nature of things, these articles can discharge their functions only after they have previously been exposed to the light of day, or some other energetic source of light.
We have in this manner become acquainted with means of altering rays of one number of waves into those of another number, and we will employ these means of rendering the ultra-violet rays visible to the eye. For this purpose, we must seek for substances possessing the property of absorbing these ultra-red rays, in order to emit them as rays of an inferior number of undulations. Besides the calcic fluoride, the above-named solutions of quinine sulphate and of esculine will answer our purpose.
We again throw a spectrum in the above-described manner, and introduce a calcic fluoride crystal into the ultra-violet part. 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. By the operation upon the quinine sulphate, therefore, the invisible rays have been converted into visible, illuminating ones.
Deeply violet-colored glass possesses the property of transmitting only the extreme violet and ultra-violet rays, and absorbing all the others. We cause a pencil of white luminous rays to emanate from the incandescent carbon-points of an electric light, which, rendered