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OCTOBER 23, 1885.J THE PHOTOGRAPHIC NEWS. 683 a metal plate pivoted to the base-board, and fastened to the back by a thumb-screw through a slot. Dark-slides are generally made so that the face of the sen sitive plate is protected from the light by a shutter or slide that can be removed to expose the plate after the dark-slide is affixed to the camera; and it is usual to have a way of introducing the plate other than through the front of the slide. The camera back used by Daguerre and Niepce, as de scribed by M. Berry (August 14, 1839), however, was solid at the outer side, the plate being introduced from the front side by opening the folding doors. When in the camera, these folding doors were opened for exposing the plate, by pushing a semi-circular arm attached to the bottom of each door, and projecting outside through the back of the camera. T. Skaife (June 10, 1856) also employs vertical folding doors, but they are self-closing by means of springs, and opened by pulling a cord that rotates a drum attached to the axis of each door. It must be a great convenience to be able to uncover and re-cover the sensi tive plate as easily as the lens is opened and closed by a shutter, when getting instantaneous views that can only be caught at short notice ; but it seems that this facility must be sacrificed in modern instruments for the sake of other more important conditions. The back described by C. Langlois (September 16, 1856) has a hinged door on each side; one is opened for introducing the plate, and the other for exposing it. The opening in the camera is done by tbe hand through a sleeve attached to a hole in the side of the camera. W. and W. H. Lewis (September 7, 1857) mention a slide that is withdrawn entirely from the camera back for exposing the plate, and the opening left is closed by a thick rod of black glass being pushed side- ways across it by a spring. The cut-off is of glass for convenience in cleaning. A. J. Melhuish (November 10, 1859) makes a very im portant improvement, which also is denied the modern operator. He uncovers the sensitive plate by drawing a slide downwards through the bottom of the camera. Now- a-days we may draw our slides out sideways or upwards at the imminent risk of moving the instrument out of position—especially if the tripod is light and its hold on the ground not firm—but never downwards. Thus, by modern methods, the opening edge of the back invariably points towards the sky, vertically or horizontally, and a focussing cloth must be used lest the light gets in as the slide is being drawn out. Melhuish’s camera back is made entirely of metal for his metal camera. J. E. Grisdale (March 12, 1862) has four hinged flaps that open back against the camera for exposing the plate ; and W. B. Woodbury (April 27, 1877) has sliding shutters of flexible metal, or of stiff metal with a part flexible, that they may be turned back when drawn out. Double-backs are described by A. J. Melhuish (Novem ber 10, 1859) entirely of metal, and by W.B. Woodbury (April 27, 1877) of wood with metal slides, and both in ventors place the plates back to back, and introduce them at one end of the dark-slide. The back is fitted into a recess in the camera, and re tained there by springs, by A. J. Melhuish (December 28, 1859), and W. F. Stanley (August 18, 1880) prefers to push the back on to a projecting tongue rather than to slide it in grooves. That sensitive plates need not be carried in the dark slide, nor in what is commonly called a changing-box, and yet be ready for use, has been shown in two or three instances of recent date. W. F. Stanley (August 18, 1880) has what he calls « inner adapters ” to the dark-slide, each carrying two plates with the sliding covers as in the usual double-back. T. Bolas (November 3,1881) carries plates singly in receptacles from which they are transferred to the camera for exposure, and then returned. The recep tacles may he roughly compared to dipping baths with sliding covers, and fit on to a suitable opening in the camera ; the transfer takes place by turning the apparatus over. J. E. Atkinson (October 4, 1883) uses loose envelopes for sensitive plates, after the principle of Stan ley’s, mentioned above, but prefers to carry the plates singly. The front of the envelope has a slide for exposing, and the back (where the plate is introduced) has a flexible flap joined to each of the four sides; the flaps are simply folded down over one another after the plate is put in. (Are the corners of the plate securely protected from light by this means ?) The back that accommodates these envelopes may be hinged to the camera, that it may turn away for the focussing screen. A back adjustable for different sized plates is described by T. Pegram (August 22, 1863), who supports the plates on opposite sides by means of two slips that slide in grooves either by hand or by screws. W. and W. H. Lewis (September 7, 1857) have a glass gutter to catch drippings from the plate when in the dark-slide, and support the plate on corners of solid glass with a rebate, instead of against the wood and silver wires as is usual for wet plates. (To be continued.) PHOTOGRAPHY OF THE INFRA-RED REGION OF THE SOLAR SPECTRUM. by william h. PICKERING. It has been generally assumed, and indeed distinctly stated by Abney and some others, that the gelatine dry plate is insensitive to that region of the spectrum lying beyond A. On trial, how ever, this proved not to be the case, as the following results dis tinctly show. It was found that there was a great difference in the plates, those made by Allen and Rowell, and those by Walker, Reid, and Inglis, giving the best results, the latter being some what better than the former. This result was indicated by experiments on the sensitiveness of the plates to daylight and gas-light—the two above-mentioned kinds being the most sensi tive of all to the latter, while only moderately so to the former light. The object of the research was to determine to how great a wave-length the plates were sensitive, rather than to obtain a good representation of the lines. A very broad slit was there fore used, and a camera lens of large diameter and short focus. The condensing lens, collimator, and camera lens were each 10 cm. in diameter, and the last of 30 cm. focus. The first two lenses were each of about 90 cm. focal length. The prism measured 10 cm. on a side, and had a refracting angle of 30°. It was so placed that the rays struck the first surface at a slightly oblique angle, thereby obtaining a dispersion equal to that which would be had ordinarily with a 60° prism, and employing only half the thickness of glass. The camera lens had an angular aperture of 19“, and the slit, as usually used, of V 20". It should be stated here, however, that in the earlier experiments made with a com mon spectroscope, using the object-glass of the telescope as a camera lens, results were obtained which compared very favour ably with those reached with the larger instrument. The prism ■was composed of dense yellow flint glass, and experiments with specimens of ordinary flint and crown glass 7 cm. in thickness gave no more absorption in the infra-red spectrum, as far as ob served, than they -would in the visible portion, which is entirely contrary to the general belief. An absorptive medium was placed in front of the slit, in order to destroy all light save that of the avave length which it was desired to photograph. This precaution is necessary, as, owing to the reflections from the surfaces of the lenses and prism, a certain amount of diffused light finds its way to the plate, together with the spectrum, and should this diffused light be of short wave-length it would fog the plate, and the spectrum image would be destroyed. It is also necessary to coat the back of the plate with black varnish, in order to prevent the formation of a halo, owing to reflections from the back of the plate. The absorptive medium consisted either of two pieces of red copper glass, or of a piece of red and a piece of blue glass, or of a thin layer of asphalte varnish on glass, of such density as to be slightly lighter than the combined red and blue glass. These all gave about equally good results, with possibly a slight advantage in favour of the asphalte. Other media were experimented upon, including red glass (single, triple, and quadruple), iodine dissolved in carbon bisulphide, lampblack, and hard rubber. The iodine solution (see diagram) transmitted a large quantity of light in the vicinity of the H lines—so much, in fact, as to