LENGTHENING THE VISIBLE SPECTRUM. 91 



Geissler's tube, the exterior envelope of which contains four phosphor- 

 escent 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. 



Fig. 2. 



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 phosphor- 

 escent 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 manufactur- 

 ing 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 ar- 

 ticles 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 fluo- 

 ride, 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 spec- 

 trum, 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 in- 

 visible 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 



