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102 THE PHOTOGRAPHIC NEWS. [Feb. 28,1862. tube, whilst the lower half of them passed through the water, and the image on the screen showed a strong contrast between the two media; the air being shown by a bright white semi-disc, whilst the water was shown as a beautiful green fluid. This colour is due to absorption. Most of the colours commonly seen are due to absortion ; that is to say, they are due to the fact that a portion of the spectrum is absorbed within the body, while a second portion is allowed to pass through the body. To illustrate this a spectrum was formed and various coloured fluids were interposed in the path of the rays forming it. Burgundy wine tested in this way was seen to absorb the blue, indigo, violet, and almost all the green whilst it transmitted the red. Permanganate of potash in solution was in this way shown to chisel out with the utmost distinctness the centre portion of the spectrum whilst it allowed the two ends, red and blue, to pass. Various coloured glasses were afterwards tested in the same way. Other ways of producing colour were then described. The spectrum itself is an example, its colours being those of refraction ; so are the colours of the rainbow, which is, in fact, a solar spectrum. The splendid colours of a thin soap bubble are not colours of absorption, but are due to interfe rence (to be subsequently explained). The iridescence on the neck of the dove, the hues of a peacock’s tail, the colours of the wings of certain insects are not colours of absorption. Neither are the gorgeous colours which were shown to be produced when colourless spirit of turpentine was poured on to pure water. The gold, straw-colour, or blue of tempered steel is not a colour of absorption. Such colours are pro duced by a thin transparent film of gas, or liquid, or solid, which itself has no colour. They may, indeed, be produced by an exceedingly thin fissure which is perfectly empty. For example the lecturer stated that he had often produced fissures in the interior of a mass of ice, which showed splendid colours, though no air could possibly reach the fissure. Colours were also shown to be produced by reflec tion from scratched surfaces. They glisten on the fine threads of the field spider. Thin clouds, especially in Alpine regions, are often flooded with the most splendid iridescences, even when the light which falls upon them is white; these colours are to be distinguished from those of the morning and evening clouds which show simply the colour of the light falling on them. Finally a dust of lyco podium, for instance, whose particles are all of the same size, when shaken in the air or on a glass plate, produces colours when a light is viewed through it. These colours have received various names in conformity with their modes of production. Some are called the colours of thin plates, others the colours of diffraction. They were all explained to depend on the great principle of interference. This, the lecturer said, was a very difficult point to ex plain, but he would endeavour to render it intelligible in the next lecture. At present he would break the ground by explaining that, although he had hitherto spoken of light as something darting out of a luminous body, as luminous particles which were reflected like billiard balls, this was not what he believed light to be. All the laws referred to in preceding lectures would follow, if light were supposed to be produced by waves or undulations, in the same way as sound. To show that sound was produced by vibrations of air, and that these vibrations actually did pass from a sounding body through the air on all sides, the lecturer performed a very striking experiment. A small gas flame was arranged, having a long glass tube placed vertically over and round it. Upon now uttering a loud musical note with his voice, the lecturer caused the flame to vibrate, giving out the same sound continuously. This proved in the most perfect manner that some of the vibrations of the air passed from the voice to the flame, and caused it to be started into vibration by hearing it. Professor Tyndall stated that he had stood sometimes at a distance of thirty feet off, and had been able to say to the flame, “ Sing,” and had then been able to say, “Stop,” and in each case it had obeyed him. DRY COLLODION: MODIFIED FROM SEVERAL PROCESSES. BY M. JANE. The plates prepared as under can be kept more than a year without losing their sensitiveness. The pyroxyline employed is a French one, which dissolved perfectly. The one made with equal parts of mixed acids at 150° Fab., I find perfectly good. When the plate is perfectly cleaned coat with collo dion :— Ether 67 parts Alcohol 33 „ Pyroxyline 1 „ Iodide of cadmium ... ... 1 „ Bromide of cadmium ... ... 0 2 „ Iodide of cadmium 0 2 „ Pure resin (colophony) 0’3 „ Put the pyroxyline, ether, and a part of alcohol in a bottle, and dissolve the iodide and bromide in the remainder of the alcohol, filter and mix them together; then add the resin. After 24 hours it is fit to work. The collodionized plate is sensitized in a bath of:— Water (distilled) 1000 parts Nitrate of silver (fused) 100 „ Glacial acetic acid 100 „ Iodide of cadmium 0-5 „ Pure iodine (French) 5 „ The plate should remain in this bath about a minute till the greasy appearance of the plate has disappeared, then re move it from the natrate bath and immerse it in a bath of distilled water (this bath of water must be changed at each plate). Coat a second plate, and when it is in the nitrate bath, remove the first plate from the first bath of water, and so on till the first plate has reached a sixth bath made as follows:— Water distilled ... ... ... 1000 parts Pyrogallic acid ... 3 „ Filter very carefully. After the plate has remained in this sixth bath, whilst another plate is coated with collodion, and to put it in the nitrate bath, take the first plate from this pure bath, let it drain and then cover it with :— Water 100 parts Dextrine ... ... ... 20 or 25 „ Camphor 5 „ Filter carefully through a sponge. This solution is made at a temperature of about 27° centi grade, or about 80° Fah. Cover twice with this dextrine solution and let the plate dry spontaneously, one corner on a piece of blotting-paper, or on two bars of wood. These plates must be kept in the same manner as plates by other dry processes. When they are to be developed—which can be done several days after the exposure—immerse the plate in a bath of distilled water for about 20 seconds, then put it on a level stand and cover it with— Distilled water ... ... ... 100 parts Pyrogallic acid } „ Citric acid 4 „ Return the solution into the developing glass, and add a few drops of a solution of nitrate of silver at four per cent, (a 20 or 25 grain solution); when intensity is obtained, wash and fix in hyposulphite of soda or cyanide of potas sium. The plate can be developed with iron by dipping, but the solution should not be poured on the plate. The iron is made as follows:— Distilled water 500 parts Protosulphate of iron ... ... 75 „ Sulphuric acid 15 drops Glacial acetic acid 10 parts. FE Int sium News RESE! TO 1 From sion sever relati used when contr iodidi I si in the rules while shall: tions, in his is plat 1st. 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