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818 THE PHOTOGRAPHIC NEWS. LDECEMBER 24, 1885. performed with such precision ; but approximately. But when we see an object which subtends at a distance of 100 m., an angle of 10°, subtending still the same angle, but apparently only 1/300 part so far off, it is set down as only 1/300 part the size, and appears in place of 176 m. in size only 25"m."* This is Herr Stolze’s chief explanation of the diminu tive appearance of the pictures seen in the stereoscope, and having given it he need, we think, scarcely have troubled himself to- give others. Another reason is given, however, which we confess we scarcely follow. We once more quote : “ Such small objects observed from so short a distance must give images vary ing from each other much more than they do when far distant. But as we are compelled, from the convergence of the rays, to place the objects so near, they appear not really solid, but rather to be in relief, and as we in turn have an idea of the correct relative distances of the objects, and seek to construct them after this knowledge, there arises a singular impression of disguise which prevents us from coming to any correct estimation of size, and strains the eyes painfully. If, besides this, we see the grain of the paper on account of the extent to which the image is magnified, it is not to be wondered at if the stereoscope, as it now exists, falls more and more into discredit, or at least becomes an instrument not commonly used.” {To be continued.) CAPTAIN ABNEY S LECTURE On Thursday evening last. December 17th, Captain Abney delivered a lecture on “Light and the Atmosphere,” at the London Institution, Finsbury. Ue commenced by saying that we are accustomed to speak of the sky as blue, and of sunlight that it is yellow, and it was upon those two points he intended to touch. In order to know any thing about light, they had first of all to decompose it and examine the spectrum produced. By means of a 50 Grove cell battery, an electric light was produced, and a decom posed beam shown on the screen. Captain Abney then demonstrated that although ordinary white light can be decomposed into seven colours, it was possible to have approximate white light which could not be decomposed into the same number of colours. By the interposition of blue glass certain cclours were absorbed, while others passed through the medium, and by still further modifica tion only blue remained upon the screen. Photometry was next dealt with, Bunsen’s and Rowland’s methods being explained ; the Lecturer giving preference to the latter as being more accurate per se, and very cheap in construction. The method was, however, in convenient for measuring the illumination of the rooms. He then introduced an instrument designed by Preece ; it was a rectangular box with a semi-opaque screen at one end, the centre of which was rendered translucent, the inside being illuminated by a small incandescent lamp. To ascertain the intensity of illumination in any given room, the light inside the box was increased until the grease spot vanished, which indicated that the illumination out side was equal to that within. The internal illumination being increased as one-sixth power of current, calculation became easy. The method of measurement by oscillation was next shown, and the extinguishing of colour by a preponderance of white light alluded to. Continuing, the Lecturer said, it had been stated that the sun was not always the same colour, but they had good reasons to think it did not vary from day today, and if they could get outside the atmosphere they would not be able to distinguish any great difference, since changes of colour were brought about by the condition of the atmosphere. At four o’clock on a September after noon the sun would appear yellow, at six it would be red ; * It appears to us that a slip in the figures has been made here. We make the apparent size only 58-6 m,m. so if tlie sun was not varialle, the atmosphere must be, and the light had to pass through a relative thickness of atmosphere which was not clear, but turbid. The theory of small water bubbles, floating in the air. next received attention, the two principal objections urged by the Lecturer being the probability of bursting, and the difference of maximum polarization, a subject which he shortly described. Professor Tyndall’s well-known experiment of passing a beam of light thiough a lurbid liquid was then illustrated by pouring a few drops of spirit varnish into a trough of water through which a beam of light was passing, which rendered a white disc upon the screen somewhat yellow. By increasing the turbidity of the liquid the colour be came deeper, the Lecturerremarking that he could in time get a sunset-red, owing to the blue becoming extin guished. Lord Rayleigh’s researches then received attention, and various colour effects were shown upon the screen to illus trate the yellow and red sunsets. Turning to the question of dust influencing the blue colour of the sky, the Lecturer thought it highly improbable that it would rise 300 feet, for above that altitude the sky was still blue ; again, dust would have to go up as it pleased, which was impossible. Captain Abney then called attention to diagrams upon the walls, and pointed out that the curves there shown gave a remarkably close reading, which pointed to its being water, and in a liquid condition. There wes no reason against the theory of water floating in the atmo sphere ; it does exist in clouds, and if we have water between us and the sun, in the form of clouds, it is quite reasonable to suppose that there are water particles in the atmosphere. Captain Abney having alluded to the glorious sunsets which were visible a short time ago, spoke of light measurement by photography, and which had given him readings at 12,000 and 13,000 feet high. The instrument was a narrow band of platinotype paper, long enough for 50 exposures, and somewhat similar to the car bon printer’s actinometer. To test the power of light at given altitudes, it was merely necessary to expose a por tion of the strip for a couple of minutes, development being deferred until a more convenient opportunity pre sented itself. The process of developing exposed strips of platino type paper was demonstrated, and it was stated that the readings taken in high altitudes showed that the actinic properties were much less than where the atmosphere diffused the light to a greater extent. Some transparencies were then shown upon the screen, and the effect of diffe rent conditions of the atmosphere on distant views pointed out, the sabjects chosen being mostly Alpine scenery, which elicited much interest. o PRACTICAL CHEMISTRY FOR PHOTOGRAPHERS BY RALPH W. ROBINSON. IV.—Sulphite or Soda—Preparation. It being so difficult to procure sulphite of soda in a state epproaching purity, some photographers may prefer to prepare their own, and as it is easily and cheaply done, 1 give for their benefit a description of the method. . \ The outline of the process is this. Sulphur dioxide 15 passed into a cold solution of carbonate of soda, until all the carbonic is replaced by sulphurous acid, thus— NajCOj+SO^Na.SOa-f-CO, Weigh out roughly a certain quantity of common car bonate of soda, or better, the “ crystal carbonate " men tioned in No. II. of these papers; but if the latter is used, bear in mind that 124 parts are equivalent to 296 parts <> the common soda, on account of the different amount < water of crystallization. Dissolve this in cold water in ■ tall beaker, or other suitable vessel. Now, into a flask fitted with thistle funnel and delie -