Few physicists to-day doubt that light consists of waves set up in an all-pervading medium called ether; that, moreover, white light is composed of different tinted rays—to be seen reflected from the bevel edge of a looking-glass, or indeed from the more natural rainbow—which further are caused by the different lengths of waves whereby the colored lights are propagated.
Now we may produce these phenomena for ourselves by cutting a slit 11⁄2 inches long in a temporary window-shutter, or, more conveniently, in the end of a large wooden box (A, [Fig. 8]). Near this a glass prism, such as once adorned gas-pendants so profusely, must be supported (B, [Fig. 8]) on a block of wood, and at the opposite end of the box a sheet of paper pinned to the inside (C, [Fig. 8]). The arrangement is shown in the diagram.
Now notice, if you regard this screen from the open top—a large cloth covering head and box in order to keep out superfluous light—a band of color is depicted thereon, gradating gently like a rainbow from violet through blue and yellow to red. Thus the white light which entered through the slit has been separated into its component parts. So far, gorgeous enough! But for the photographer much greater interest is at hand, will he proceed as follows. Place the box so that bright sunshine enters through the slit, and after fastening a piece of blue print paper instead of the white on the box interior, allow this to print, at the same time excluding extraneous light by a dark cloth overspread. When this piece of paper has been washed and fixed, the colored band should have registered itself in various shades of blue, from which it will be noticeable that the purple and blue lights have darkened the paper most, whilst red and yellow hardly affect it at all. If ordinary printing paper, or still better, a photographic plate (in which latter case a considerably shorter exposure will suffice, and outside light must be rigorously excluded) be used, instead of the blue print paper, the result is still more striking.
Fig. 8.—The refraction of white light into its constituent colors.
From this experiment further is to be gathered the reason for developing plates by red light, which evidently does not affect the sensitive surface in any appreciable degree. But, on the other hand, special orthochromatic plates are made which, by dyeing, have been rendered sensitive to the yellow rays as well as to the blue, and if one of these be employed to “take” the colored band, technically called “spectrum,” a totally different gradation is obtained compared with that on an ordinary plate or on “blue print” paper.
Then again, suppose instead of sunlight, ordinary lamplight or incandescent gas be used as an illuminant, the gradation varies, whilst still another modification is to photograph the spectrum of a methylated spirit-flame in which common salt is being burnt. In this case the light is so yellow that an orthochromatic plate must be used. Another illuminant worth testing is magnesium ribbon, which also may be ignited in the spirit-flame.
Before saying a final adieu to these spectrum results, one last item remains for remark—last but not least. We say that the series of visible colors extended from violet through blue and yellow to red, and that darkness obtained at each end. Well! Mount a plate or piece of printing paper inside the box, so that half of it is well in the darkness beyond the bluish-violet bands, and expose long enough to secure a slight opacity in these parts (i.e. where the blue bands fell) when the plate is developed and fixed. The half of the plate which was in darkness and therefore apparently received no exposure, nevertheless develops darker than the remainder, seeming to indicate that some light, although invisible to the human eye, had affected the sensitive silver compounds in the plate. This is actually the case. The rays of light which exert this wonderful influence are called “ultra-violet,” meaning beyond the violet, and their existence explains, amongst other matters, why photography of the heavens has revealed the presence of many thousands more stars than can ever be seen by man. Such stars emit only “ultra violet” light.
Experiment E.—One Person in Two Places—and Spiritualism
Pictures of a man decapitating himself, or of the reader’s sister turning the skipping-rope for another girl, who is herself, may justly be called mystifying. Not only may they almost deceive the operator himself, but will quite nonplus the uninitiated, to whom proofs may thus be presented of the most impossible happenings. Two methods are applicable to the production of such freak portraits, viz:—