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Feb. 21, 1862.] THE PHOTOGRAPHIC NEWS. 89 is capable of reflecting back is the red. The same fact was afterwards illustrated with coloured sealing wax. Mr. Anderson, the assistant, (possessing a tolerably ruddy com plexion,) was then requested to pass along the rays of the spectrum from the red upwards. At first his face shone with the bloom of youth ; as he passed towards the yellow, one side of his face assumed a very cadaverous aspect, which became perfectly frightful when he entered the blue rays. Upon returning, one side of his whiskers became quite radiant, whilst the other side of his face was corpse-like; until, upon fairly entering the red ray, he shone in the full splendour of boyhood. In fact, the colours of bodies are entirely due to the light which falls upon them. If the white light of the sun were simple instead of compound, we should only have light and shade in the world, but there would be no colour. The beautiful spectra produced by the combustion of various metals, was afterwards exhibited. A metal heated to whiteness gives a continuous spectrum as long as it re mains in the solid or liquid condition; but when the metal has been reduced to vapour, and when that vapour is ren dered luminous by intense heat, the spectrum of the vapour is usually composed of brilliant bands. Every metal has its own distinct system of bands. These were beautifully exhibited in the case of cadmium, copper, and zinc, by placing the respective metals on the lower carbon pole of the electric lamp, and refracting the light so evolved by means of the usual arrangement of prisms, lens, and slit. The bands produced by copper and zinc having first been exhibited, those of their alloy, brass, were shown, by which it was seen that the bands of each metal are produced in the spectrum of the alloy. Thus, knowing the bands which each separate metal produces, we can determine from the spectrum of an alloy the metals of which it is composed. The bands of metals were also shown to be produced when the salts of the metal are raised to a sufficiently intense tem perature. The light evolved by the ignition of the vapour of the metal sodium was then exhibited. This is intensely yellow, and gives a spectrum consisting of one single yellow band. This was first shown to be the case, and it was then ex plained that a luminous vapour absorbs those rays, which it can itself emit. Thus the light of incandescent sodium vapour is intensely yellow ; but when a beam from the elec tric lamp is sent through this vapour, the yellow rays of the electric light are intercepted. This was shown by first projecting on the screen the continuous electric spectrum, and then interposing the vapour from a piece of burning sodium in the path of the rays. A broad black band was instantly ploughed out of the spectrum in that part where the yellow soda line formerly fell. The recent theory of the composition of the sun was finally briefly examined. The sun, according to this, is supposed to be composed of a solid or liquid central portion, which of itself would give a continuous spectrum ; but this nucleus is surrounded by a flaming atmosphere, through which the rays from the nucleus have to pass. This solar atmosphere, or photosphere as it is often called, intercepts those rays of the nucleus which it can itself emit, and hence 'the solar spectrum is always furrowed by dark lines {Fraiienhofer's hues). From these lines we can determine the metals which Produce them I and in this way it has been found that many oi the terrestial metals are present in the sun. REMARKS UPON SOME OF THE APPARATUS EMPLOYED IN PHOTOGRAPHY* BY F. It. WINDOW. CAMERAS.—Continued. The Focussing Screen.—It has always appeared to me a mistake to make the focussing screen a separate piece by itself, instead o attaching it permanently to the camera. * Continued from page 74. It not only has to be put into the instrument and taken out again every time that the focus is examined, but when taken out it has to be laid aside, generally in a hurry, and it runs a great chance of being broken. This applies to all descriptions of cameras, though the remedy cannot in every case be effected in the same manner. In cameras for the studio it may without any inconve nience be hinged at the bottom, and, opening outwards, it will lie, when not in use, upon the top of the camera stand if this latter is larger than the camera, (fig. 5), or it will hang down parallel with the back if the camera projects beyond the stand, (fig. 6). In either case it will be out of the way of the plate holder, and the arrangement offers no practical objection, while it cannot be denied that it is much more convenient than the usual detached focussing screens. When constructed in this manner, the frame of the screen Fig. 6. will not enter the grooves which receive the plate holder, but will lie flush up against them. It is kept up in its working position by means of a spring button on the back of the camera. There many other ways in which the focussing screen can be constructed to form a permanent part of the camera, but I have described this one in preference, as it is the most generally applicable, and the simplest. The ground-glass which is now supplied for the best focussing screens is of a very superior description to that which we had to use at first. The grain is fine and even, and the image appears upon it smooth and distinct in the details. Still I am inclined to think that we should find an advantage in using a polished glass, flashed with an extremely thin layer of' opaque white glass instead of the granulated surface : for this would be as good or better than the ground-glass for ordinary purposes, and a high magnify ing power might be used to focus subjects offering much minute detail, or the negatives of which were intended to be subsequently enlarged with the solar camera. I am not aware that there is any obstacle to procuring glass flashed in this way ; if not, I am inclined to think its use should be recommended, as just now small portraits re quiring minute care in focussing are in vogue; and the time is not far off when enlarged pictures from small original negatives will be raised to their proper place in public estimation, and it is essential for these latter, which are to be looked at magnified, that the operator be able to assist the eye with a magnifier while arranging the focus. {To be continued.) ——