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Feb. 14, 1862.] THE PHOTOGRAPHIC NEWS. to the table, and, therefore, there is no chance of disturb- ing the focus by putting it in or taking it out, which, if the body be not well clamped down, sometimes occurs with the ordinary camera. The addition of the racks and pinion, of course add a little to the. expense of the camera, but they replace the guide bar, clamp, and screw of the old system which dimi nishes this extra cost. In any case this form is not so expensive as the rack and pinion adjustment to one lens, it is more practically efficient, and one camera so fitted will serve for all lenses, which may be mounted in a simple rigid tube. Io the purchaser of the complete apparatus con sisting of camera and lens, this form would be cheaper than the usual design of camera with a lens having a rack and pinion motion. The endless screw movement for advancing the inner body, or for any purpose of focussing is open to objection. The movement is too slow, and the eye has great difficulty in appreciating the exact value of the gradually changing image on the screen. The precise point of focus is mostly estimated by contrast with positions a little on cither side which appear less perfect; but if this contrast is brought about very gradually the power of appreciation is less sensi tive. Those who are accustomed to use the microscope well know how difficult it is to get the exact focus with the fine adjustment. The inside body of the camera is always made to enter flush into the outer body, depending upon the exactness of the fitting to exclude all light, as shown in the section, (fig. 3). This is a fault, for two wooden surfaces cannot well make a light tight joint, unless they are kept by pressure in intimate contact. This is the cause of much inconvenience ; of light getting into the camera, or of the bodies not work ing smoothly one in the other. The former is the more common consequence, but it is the most difficult to detect, and to remedy. The danger of light entering the camera becomes greater as the inner body is more pulled out, and it is never safe Hig. s toremploy t 0 the utmost the powers of extension. A piece of cloth or velvet is sometimes placed upon one or both of the surfaces to make the joint more light tight, but this produces as great an evil as the one it is intended to correct, the friction soon partially wears these substances, and every movement of the body inward or outward scatters their dust about the inside of the camera. A proper way to make this joint is to run a strip of wood one-eighth of an inch thick, and a quarter of an inch wide, round’the inner edge of the outer body, and also round the outer edge of the inner body, as shown in fig. 4. In this manner light is entirely exduded without the necessity for a very tight fit, or recourse to packing of any description The frictional surfaces also are reduced to a trifle and movement is rendered easy and regular. This plan of adjusting the inner to the outer body decreases the dimen- sions of the former by a quarter of an inch, which if carries the lens, as it ought, is of no consequence ; but it it is made to carry the plate-holder, the difference must bi allowed for by increasing the outside body by so much. ‘ The front of the camera which holds the lens should h vo a vertical movement in order that the image upon .' screen may be brought into the required position wit altering the level of the camera. This is usually mad' that when the lens is in the middle of the instrument, 1 moveable front is at its lowest point, and allows, theref n only of an upward motion. For (this reason it is called a “ rising front.” But, as it as frequently happens that 11 ■ image requires to be lowered, as to be raised, the moven should be contrived so that the lens is in the centre of tho instrument, at the middle point of the motion of the fro by this means both an upward and downward movemen at command. The amount of vertical movement that can be given to the camera front depends upon the nature of its const ac tion, but in all cases it should be as much as possible. A camera for the studio, where size is of no object, she aid be considerably larger in all directions than the largect pie - ture it is intended to take; and a diaphragm should be pla i about an inch and a half in front of the plate, with 11 opening just sufficient, and no more, to give passage to the direct rays from the lens on to the plate. This is seldom found in the cameras ordinarily offe ■ d for sale ; but it is very important, as it shades the p t • from much of the light which would otherwise reach it ! y reflection from the sides, top, and bottom of the camera. ' t must be remembered that while a picture is being taken he camera is full of white light, which, coming in through lens strikes directly upon the surface opposite, that is, he prepared plate, and (if there be no diaphragm) the polis. 1 ■<' plate-sldie; and also, if the camera be narrow in pro] tion to the angle of the range of the lens, upon the sur rounding portions of the top, bottom, and sides. The direct rays are reflected upon other parts of the camera, te ultimately are returned upon the sensitive plate. These effects will evidently be diminished by the use of such a diaphragm, as I have described. Properly, th a diaphragm, which is a fixture, should have fitted to it carriers, or moveable diaphragms adapted to each size , plate that the camera is intended to employ. The fact of the camera being deeper and broader than the glass plate does not necessitate the plate-holder being larger in the same proportion; as, if it is considered advisable, the grooves to hold this latter can be constructed to hold slides of the ordinary dimensions. It is not necessary to state here that every part of the inside of a camera should be black; but it is as well to remark ——that a perfectly dead black should be em ployed for this purpse. I have seen a large number var nished brilliantly in the interior with some black spirit varnish. There is an excellent dead-black varnish in the market, wich every one can use for himself for this pur pose, to which it is well suited. It has been proposed to lino the camera with black velvet. No surface could be more perfect for the purpose, as it is unequalled for its non-reflective properties; but it would be a source of dust and dirt, and for that reason I do not think it is to be recommended. A shade to protect the lens from all side and top light must be attached, outside, to the front of the camera. This is usually made as if it were a continuation of the four sides of the camera, and projects forward considerably beyond the lens. Care must be taken, however, that it does not project sufficiently far to interfere with the range of the lens, or- to offer any inconvenience in taking off and putting- on the cap. There is clearly no necessity for the dimensions