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August 3, 1883.] THE PHOTOGRAPHIC NEWS. 491 a plentiful application of camera-stand, we made a little progress, and, once more coming in sight of the sea, we found an easier road. Now the guide thought we ought to gallop, but the horse thought different, and, being desir ous of munching a little grass, came to a stand. Another stand (the camera-stand) came to him, and pretty briskly, too, after which he went for another half-mile, and turned round as if he would like to go home. This was too much. Then I bethought myself of the guide’s spurs, and proposed changing horses. As it was now getting late, and my argument was persuasive, this was agreed to. My new horse had only been broken in three months, and went like the wind. So did the other, for sharp spurs are more persuasive than camera-legs. On we went through the forest again, over softer ground, and occasionally through bog. At last we found ourselves less than two miles from the Volcano House, and the road was good. Bringing the horses to a smart gallop, and keeping a good seat, we dashed over the ground, for I meant to get in before dark, if I broke my neck over it. Reaching the house in safety, I was soon in the midst of dinner, and that performance concluded, I took stock. Results:—General stiffness and divers sores, aching shoulders and sore back from my camera, broken ground-glass, and a few plates smashed. ##♦*** I can’t help it, Mr. Editor. Please don’t! Go to Hilo and administer it to those horses ; some of them want some physic. 1 know there is nothing about photography in this letter, but I brought in the camera legs as often as I could, though not as often as I did on the road. After all, this is a very good article on “ Where not to go with the Camera.” {To be continued.) & Bictionary £lf Photography. "MM. FAYRE and Silbermann have examined the action of the solar light on a mixture of chlorine and hydrogen, and have employed the facts which they have observed in measuring the chemical influence of the different coloured rays of the spectrum. They filled fifty small glass tubes, placed in an upright position along the sides of a narrow trough—the gases being confined over salt water, whilst they were exposed to the solar spectrum. The level to which the salt solution rose in the various tubes under the influence of light rendered the chemical action of the coloured rays evident to the eye. Favre and Silbermann state that they have found the greatest action to take place in the morning at the line H, at noon at the line G, and in the evening at F. “M. Claudet has also devised an instrument which he terms a photegraphometer, by means of which we are enabled to measure, not only the intensity of the chemical rays, but also the relative susceptibility of the plates or chemical papers, which have been prepared according to different methods. The plate or paper is attached at the lower edge of an inclined plane, and covered with a metallic plate, which is perforated horizontally with a row of equi-distant round holes. A second disc slides along the inclined plane, in which there are corresponding holes of 1, 2, 4, 8, 16, 32, and 64 millimetres in diameter. This second plate is fixed in a black cloth, which moves with it, in such a manner that the rays of light can only impinge upon the prepared plate during the time which the openings of the moving plate occupy in pass ing over those in the one at rest. It is evident that the ratios of the periods of operation for the adjacent perforations must be as 1, 2, 4, 8, &c. When a very weak light is employed, as in instituting a comparison between the solar and lunar light, it is necessary to let the movable plate fall repeatedly, and to calcu late the ratios of the intensities accordingly. If we wish to com pare plates that have been prepared according to different methods, they must be placed in juxtaposition, and two movable plates allowed to slide down at the same time, as the intensity of the light varies every minute. “Professor Draper has devoted considerable time to the study of the chemical action of light; and many years ago he commenced experiments, with the view to invent some means for measuring the chemical action of light with some degree of exact ¬ ness. His first essays were by noticing the degree of blackness which was produced on papers coated with chloride or bromide of silver. He subsequently described an instrument which was well adapted to these enquiries. This he described in a paper published in the Philosophical Magazine; and it is from this, and other papers by the same experimentalist which he has communicated to that journal, that the present account of his researches is taken. The instrument, to which he has given the name of tithonometer, consists of an arrangement by which there may be obtained from hydro-chloric acid by voltaic decomposition a mixture of equal parts of chlorine and hydrogen. This mixture will remain with out change in the dark, but on exposure to the rays of a lamp the two gases unite in proportion to the incident light. So great is its sensitiveness, that an electric spark which lasts, it is said, less than the millionth part of a second, affects it powerfully when at a distance, and sometimes occasions an explosion which destroys the tithonometer. Messrs. Bunsen and Roscoe have introduced several improvements and refinements into the tithonometer; and in a paper which they read before the Royal Society, they have brought forward many important discoveries in photo-chemical science which they have been enabled to make by means of this instrument. They belong too much to the domain of abstract science for us to lay them in full before our readers. The apparatus which these physicists have contrived for this purpose is most ingenious; and, although too complicated and delicate for any other purpose than an instrument of pure research, promises to be of the highest importance in all inquiries into the laws which regulate photographic phenomena. Messrs. Bunsen and Roscoe have obtained several remarkable results with their instrument; one of them is, that the presence of a very minute quantity of a foreign gas introduced into their standard mixture of chlorine and hydrogen was capable of offering great resistance to the combination of the gases, a small quantity of hydrogen in excess diminished the sensitiveness by two-thirds, whilst a little more than one per cent, of oxygen almost entirely prevented combination. Their researches have also shown that the obser vations of Becquerel, which induced him to assume the existence of certain rays which could continue but not commence chemical action, may be explained without having recourse to the hypo thesis of the existence of a new property of light. They have also discovered a very important law governing the chemical com bination of a mixture of chlorine and hydrogen, and which it is reasonable to suppose equally well applies to the other cases of combination or decomposition induced by the agency of light, which is, that ‘for a given amount of chemical action, effected in the chlorine and hydrogen, an equivalent quantity of light is absorbed.” These experimentalists have also noticed that the chemical rays from various sources of light are very different in quality, and that the chemical rays reflected at different times and hours not only possess quantitative but also qualitative differences, similar to the various coloured rays of the solar spectrum ; and they conclude their elaborate paper by a reference to the influence which these qualitative differences in the chemical rays exert on the photo-chemical phenomena of vegetation. They state, ‘ that this influence must be of the greatest importance is evident from the varying effects produced in other photo chemical processes by differences in the solar light.’ We must only mention, in proof of this assertion, the fact well known to all photographers, that the amount of light, photometrically speaking, gives no measure for the time in which a given photo chemical effect is produced, and that a less intense morning light is always preferred for the preparation of pictures, to a bright evening light. “ Prof essor Draper has also suggested another means for'measur- ing the chemical action of light, and one which will be found well adapted where extreme sensitiveness is not desired. It is by employing an aqueous solution of peroxalate of iron. This sub stance is of a golden yellow colour, and may be preserved unchanged for years if in total darkness, but on exposure to the light of a lamp, or to daylight, decomposition immediately takes place, and a lemon-yellow precipitate of protoxalate of iron falls down, with evolution of carbonic acid. « The rays which chiefly affect this solution are the most refrangible indigo and violet rays—the same, in fact, which affect the tithonometer and silver salts in general. In its application to photometry several plans may be pursued:—The quantity of the carbonic acid produced may be estimated either by determining its weight or volume ; or a determination might be made of the weight of certain metals—gold or silver, for instance—which the solution after exposure would precipitate. “ Several precautions must be borne in mind in experimenting