INVISIBLE RAYS, PHOTOGRAPHING BY The fact that photography can depict objects invisible to the eye has long been known. Fox Talbot referred to the subject in his book, " The Pencil of Nature," published in t844, and his own words are " Among the many novel ideas which the discovery of photography has sug gested is the following rather curious experiment or speculations. When a ray of solar light is refracted by a prism and thrown upon a screen, it forms there a very beautiful coloured band known by the name of the solar spectrum. Ex perimenters have found that if this spectrum is thrown upon a sheet of sensitive paper, the violet end of it produces the principal effect, and, what is truly remarkable, a similar effect is produced by certain invisible rays which lie beyond the violet, and beyond the limits of the spectrum, and whose existence is only revealed to us by this action which they exert. Now I would propose to separate these invisible rays from the rest by suffering them to pass through into an adjoining apartment, through an aperture in a wall or screen. This apartment would thus become filled (we must not call it illuminated) with invisible rays, which might be scattered in all directions by a convex lens placed behind the aperture. If there were a number of persons in the room, no one would see the other ; and yet nevertheless if a camera were so placed as to point in the direction in which anyone was standing it would take his portrait." More than half a century later Edgar Senior, of the Battersea Polytechnic, took a most successful portrait under what are practically those conditions. The source of dark (invisible) rays was an arc lamp, the visible light being cut off at the lens by means of special screens invented by Prof. R. W. Wood ; the necessary exposure was five minutes. The " X-rays " discovered by Prof. Röntgen, of Wurzburg, in 1896 are invisible rays, and the work they will do is common knowledge. (For " X-ray " work, see under its own heading.)
The photographic spectrum therefore stretches out beyond both ends of the visible spectrum, and measures seven or eight times the length of the visible spectrum. Thus, not only do ultra violet rays give results photographically, but the infra-red as well, although the latter are, of course, at the opposite end of the spectrum and below the visible red.
Prof. Sylvanus Thompson, at the Royal Institution in 1896, illuminated a piece of apparently white paper by means of a arc-lamp. A photograph was then taken of the white paper, and the negative showed an inscription written thereon. This inscription had been written upon the paper with an acid (citric or sulphuric) solution of sulphate of quinine, which, being like water in appearance, could not be seen by the eye. The camera detected it, however, because the chemical liquid absorbed the ultra-violet rays, and they were not reflected to the plate, hence they appeared black. The experiment may be made by anyone, but it is important that a wet collo dion plate be used and not a modern dry plate. Dr. Gladstone made similar experiments as early as t873, and exhibited his results at the Brad ford meeting of the British Association in that year.
There are many substances that are fluorescent, or that change the refrangibility of rays of light, and which have a light action upon a photo graphic plate. An unlighted incandescent gas mantle gives off sufficient invisible rays to make an image upon a photographic plate. Among other substances are radium, mineral uranite, certain salts of uranium, canary glass, alcoholic solution of chlorophyll, aesculin, tincture of stramonium seeds, and of turmeric.
Prof. Wood, of the Johns Hopkins University (U.S.A.), has paid particular attention to the action of the invisible ultra-violet and infra-red rays, and his lecture before the Royal Photo graphic Society on September 27, 191o, should be referred to for full particulars.