The Optical Magic Lantern Journal (October 1893)

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144 The Optical Magic Lantern Journal and Photographic Enlarger. trum the deflections of the galvanometer decline through red, orange, yellow, green, blue, and i | violet light in succession, until the other end of | the visible spectrum is reached. The radiations | and the heat, however, do not cease there ; the spectrum at this other end is also invisible, but has little heating power, although its chemical and photographic influence is more or | less strong. The presence of these ultra-violet j rays may be made evident by means of fluorescent substances, such as sulphate of quinine or fluorescine. These take up motion from ultraviolet invisible rays, and degrade them so as to throw off rays of lower refrangibilty, which are visible. The atoms in a condenser and in all other material bodies are believed to be in a state of | motion, in a state of linear, circular, elliptical or other vibration, the exact nature of which is not known. Because of this motion solid bodies are continually changing their dimensions with variations of temperature. A poker, for instance, varies in its length with every change of its temperature. These molecular motions are exceedingly powerful; if a condensing lens have too tightly fitting a mount, when the former gets heated it will either crack itself or tear the mount with the pressure of molecular expansion. Water in freezing into ice expands then placed in a pail containing plenty of a freezing mixture, say of ice and salt, when the water inside the bomb freezes it will burst the bomb with a dull heavy thud, breaking the shell perhaps into two or three pieces. The molecular power of contraction is also great. When the walls of a house have been bulging out, they have been pulled back to the vertical again by the contraction of long iron rods first heated and screwed up tightly to iron plates cutside the house, then allowed to cool, so that as they shortened by cooling they drew the walls together. Why is light not much obstructed by the glass of a condenser? The prevalent hypothesis is, that when the vibrating molecules of matter in glass move with the same velocity of vibration as the incident waves, those particular waves do not get through, but increase the motion of the molecules: in other words cause the glass to get hot and increase its bulk, and this is what is done with dark radiant heat. Other radiant vibrations, however, are not synchronous with the vibrations of the molecules in the glass, and these waves in large part pass between or round the molecules, and come out on the other side, so that while the dark rays are heating and enlarging the condensor, the rays of light find their way through. Glass is tolerably opaque to radiant heat, so absorbs it and becomes heated. Rocksalt is perhaps the only solid substance : transparent both to radiant light and to in, visible radiant heat, hence a lens made from a } rocksalt crystal will not get anything like so hot as a glass lens; the former lets the heat rays pass through instead of itself absorbing their energy. This property is of no use for the magic lantern, for the intense heat had better be chiefly arrested by the condensor rather than to pass through it to be arrested by the slide. Moreover, rocksalt is hygroscopic, and those who are fortunate enough to possess fair-sized plates of it worked from good crystals , Store them in closed vessels along with some substance for keeping the air dry, such as under a bell jar, and supported over a saucer containing sulphuric acid, which is Professor Tyndall’s method of storing his specimens. He discovered that a solution of iodine in bisulphide of carbon, although opaque to light, is transparent to dark radiant heat, so that by filling a cell with the solution, the cell having rocksalt plates at each end, he could cut off all the light from the electric lamp, yet in the : darkness outside the cell set fire to brown with such force that if a bomb-shell be com| pletely filled with water and soundly plugged, | paper. The rays were brought to a focus outside the lantern by means of a suitable reflector placed behind the electric are, whilst the iodine solution prevented the visible rays reaching the focal point. After one piece of glass of sufticient thickness has absorbed most of those rays which heat glass, the transmitted rays sifted by this first | piece of glass will exercise much less heating power upon a second piece of glass, and’ so on in succession ; thus the piece next the radiant has to stand the great brunt of the heating effect. Unfortunately this piece must be thin, otherwise it will be more liable to crack under the rapid application of heat, and it may not be thick enough to exercise sufficient absorbent power. With triple condensers perhaps it might be found advantageous to use two or three flat pieces of thin glass with a small space between each, and to place all of them between the radiant and the condensor. There should be sutticient ventilation in the mount of these to carry off the hot air. That a certain thickness is necessary to get complete selective absorption of radiant energy, may be seen in the case of coloured glass. If any piece of stained glass be sufficiently thin, it is practically colourless, but as more of these pieces of