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66 [January 30, 1885. HEAT MEASURERS AND PHOTOGRAPHY. It is very often forgotten, though for some purposes it should be remembered, that when light passes through different media, such as coloured glasses, not only are the visible rays cut off, but that in the ultra-violet and ultra-red part of the spectrum there are rays which have also to be taken into account. It is more particularly the ultra-violet rays which must be reckoned with by photo graphers. If a thermopile of a proper shape be moved along a solar spectrum, or that due to artificial sources, the sensitive galvanometer which, by the amount of the deflection of the needle, indicates the heating of the thermopile from the different radiation, tells us that very little heating power or energy is existent in the ultra violet or blue end of the spectrum. When the pile is in the yellow, there is a decided deflection of the needle, and as it enters into the red, there is a still greater deflection, which attains its maximum far beyond the limits of the visible spectrum, and then lessens till at length there is no indication of any radiant energy whatsoever. We may remark that the heating power of radiation is an exact measure of its energy, and a thermopile is an excellent way of making comparative measures of the heating power of the heat, as it can oe made to indicate a rise in temperature of as small a quantity as 1060 of a degree in the lamp-black with which the face of the pile is covered. Captain Abney has at various times shown that certain kinds of yellow and orange glass are permeable by part of the ultra-violet spectrum ; and in a paper read recently before the Royal Society by him and Colonel Festing, they have attacked with success the ultra dark part of the spectrum, and their results have a certain amount of interest for photographers. It will be in the recollection of our readers that Captain Abney prepares a collodion emulsion with which he is able to photograph a kettle of । boiling water by its own radiation—that is, by the dark | rays which radiate from lamp black. Now, in analysing these dark rays with a prism, they are so crowded together I that in the case of solar radiation it is impossible to dis tinguish in a photograph impressed by them anything beyond general indications that the earth’s atmosphere, or that of the sun, absorbs certain masses of them ; and it was hopeless to find any similar fine lines to those which are called Fraunhofer lines in the visible part of the spectrum. 1 In order to ascertain the existence of such, it becomes necessary to use a diffraction grating, by which the dark rays are as much separated as the visible. Unfortunately, I with a diffraction grating there are what are termed I different orders of spectra on each side of a central image of the slit through which the light to be analysed passes. ' The two spectra nearest the image or the slit are called | spectra of the 1st order, those next of the 2nd order, and so on, and the ultra-violet of the 2nd order overlaps the ultra-red of the 1st order, whilst the blue of the 3rd order overlaps the red of the 2nd. It is evident that if a sensi- 4 9? 8 visible. 8 invisible. imsuiiHtaBti mmBBBMMMMHM NiiaiBmiiBiiiiiiHiB msuyanvmnumm IBBBBBBBEmBBKBBBBBBBBB RBBBBBBIBmBIBBiBBflBBBI IBBflBBBIBilBMBEBBBBBB RSSBBBBSimmiBEBBflE AKErbggessnapaevamesm The sc alenumber means tenths of an inchin the height of the spectrum. The height ofthe’kurvesare relative h e ating effects." tive plate were placed in the spectrum of the 1st order, th e plate would be impressed by the ultra violet of the 2nd order, and that the photograph would therefore be value less if it was required merely to photograph the ultra red rays. One'way, though there are others, of getting over this difficulty is to place coloured glass before the slit of the spectroscope, such that all the rays from the ultra-violet to green, yellow, or red, are absorbed, and then to photo graph this filtered spectrum. The question, then, is, what medium is the best to employ ; it being an essential that the rays to be photographed should be as little affeeted as possible, whilst the other rays should be cut off. To this question Captain Abney and Colonel Festing have given a decisive answer. The source of radiation they em ployed was a small glow lamp kept at a temperature of ' about 1650° by means of an electric current. After the radiation had passed through various media, thermograms of the spectrum were recorded by means of the thermo pile. The annexed diagram will give an idea of the result 3 No. I curve is that of the unshaded lamp, showing no absorption of any special kind, as would be expected- No. II, that of orange glass, the orange being obtained by flashing one side of the glass with oxides of silver and antimony, and the other with cuprous oxide. No. Ill is that due to ruby glass, whilst Nos. IV and V are those given by the absorption of cobalt and green glass respectively, the latter being obtained by flashing with a mixture of th® oxide of iron and cupric oxide. It will strike even th® uninitiated that orange and ruby glass are far better media for the purpose in question than cobalt or green glass)