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74 THE PHOTOGRAPHIC NEWS. [Feb. 14, 1862. slight loss of silver when the metal is dissolved in nitric acid in the ordinary way. The following modifications were therefore introduced into the mode of operating. The silver was introduced into a white glass flask, with the stopper well ground in with emery, and having very thick side . s; as : enab it to withstand an internal pressure of at j • st ten a mos es. On this metal was poured about -n times its weig t ol pure strong nitric acid. The stopper a tin, a d red strongly in its place with thick cord The flask then closely surrounded with thick wire gauze, and i la d in a bath where the temperature couldb raised to 1 to 50° C. At the end of twenty-four o> thirty-six hou .. all the silver is dissolved quietly, like sugar would dissolvo .in water, without a trace of gas de- rel - If elf, ■ id without anything escaping from the ' . ... Indeed, Che i inoxide of nitrogen as fast as produced red s the nitric . 'd into the state of nitrous or hypo- r> . . aci is. w] ich it this temperature remain perfectly tiss Iv I in e 1 rge excess of nitric acid employed. If b ■ mperatur • f the bath does not exceed 50°, there is ally nothing t( • a indeed, M. Stas states that he has ma le under these > editions, and without any accident taking place upward of a hundred solutions of silver in a clo ed fl ask, employing from three to fifty grammes of silver at a tine. Twice only, the temperature of the bath rising much too high, did two flasks, which were immersed in it, yield ti the internal pressure, and produced a rather violent explosion. It is probable that this process of dissolving silver in ; trie acid could be advantageously employed on a large ■ale in manufacturing the salt for commercial purposes. 1 here is an appreciable quantity of silver carried off by the gas and acid vapours evolved when the metal is dissolved i i nitric acid in the ordinary way, and this, when we take into consideration the large scale upon which manufacturing operations are conducted, would in time amount to a con- siderable quantity. Dissolving the metal in closed vessels woul I entirely obviate all danger of loss from this cause, and would likewise be of use in another way. Whilst the metal is being dissolved with free exposure to the air, there i always the danger of—nay, the impossibilty of avoiding —the absorption of floating atmospheric organic matter in the. acid; but when the solution is effected in a closed ve sei, there is a chance of the nitrate being less contami- nat d, inasmuch as the most critical part of the operation has been performed out of contact with the air. We have to report upon three samples of illuminating media a the dark room. One is a brown glass of a rather dark our, s nt by M. A. It cuts off all the active rays, ■ "1’the upper green and lower blue. In a dark , si' ’ ’ a d with an iodised collodion, it would answer very - • would not be safe to use under all circum- ; stances. also ■ nds a piece of varnished, yellow ; muslin. Th w ■ ■ fue rays comin g through ■ ■ quantity. W. L. likewise sends a pieie of ali co prepared with f r I coats of glue and 3-grai f nitrat cl of, solution. This is far infer io both i ..... aabsorp-1 tive properties to those we rec ■■ bor" It allos s abundance of active rays to ■ nd a examination with the naked ey sh wsth, i 4 ith minute holes admitting white light. J is, thereore’quite unfitted for the purpo ; for whick it intend d REMARKS UPON SOME OF THE APPARATUS ■ED IN PHOTOGRAPHY* bv r. b. WIdow. C i M ERas. — Continued. The Studic ' a . The office of a camera is extremely simple. It is to si ppo ri the lens and the sensitive plate, at a proper distan part, in such a manner that no ray of * Continued from page 53. light shall shine upon the prepared surface of the latter, excepting those passing directly through the former. The studio camera consists of the body, generally formed in two parts, called the outer and the inner body ; the focussing-screen, and the plate-holder or slide. The Camera Body.—It has been remarked above, under the head of lenses, that a double-bodied camera is to be pre ferred for portraiture, the focal distance being adjusted by sliding one body within the other. The ordinary form of Camera is not very conveniently arranged for this purpose. The outer body, a, (fig. 1) carries the lens, and is fixed on to the table or base. The inner body, b, sliding upon this table carries the plate-slide. In the slot c, in the table, Fig. 1. runs a metal rod attached to the inner body, which serves more or less to preserve the parallelism of its motion, and, by means of a screw, d, and a nut underneath the table, can be clamped tight, retaining the sliding body in any required position. Thus, the only means of obtaing a correct focus of an object with this camera is to take hold of the projecting portion of the inner body with both hands, and push it in wards or pull it outwards, as the case may require; and when the desired position is attained clamp it with the screw. This is so inconvenient in practice that it has almost necessitated the use of a lens having a rack and pinion, with which the exact focus is ascertained, after having ob tained it approximately by moving the body. I lately suggested to one of our first opticians a movement for the sliding body, and an arrangement of the parts, which he at once adapted to all his cameras; and I per ceive that many houses are already beginning to adopt the same design, which I will describe, as I believe it is a con siderable improvement upon the old form, which I have just given. Fig. 2. In this camera the body, a, (fig- 2) fixed to the table, carries the plate-holder, and the moveable body, b, carries the lens. To the bottom of the front of the moveable body, b, are screwed two racks, c and d, one at each side arid within half an inch of the edge, These racks, parallel to each other, lie upon the table of the camera, and both work in one pinion, e, terminating in a milled head,f. By turn ing the milled head the sliding body is moved backwards and forwards smoothly, and the parallelism of the motion is assured with a certainty which could not well be attained in any other way. This quality is especially valuable for binocular stereoscope cameras, or for carte de visite cameras in which two or more lenses are used simultaneously. Another advantage possessed by this form of camera is that th plate-holder i in th- nortion of it which is fixed
