The ordinary double plate holder is perfectly safe when at rest, but as soon as the slide is put in motion it is not light-proof, from the very nature of its construction. Any one will readily be convinced on this point by drawing the slide in full sunlight.
Nearly 2 years since it occurred to me that a more certain and convenient method of exposing a dry plate could be devised, and a rough model was then constructed similar to that about to be described.
The plate holder (Fig. 240), shown partially open, consists of a rectangular box b, made of cardboard, tin, or brass, preferably the former. In the bottom of the box is glued a strip of wood 1/2 in. in width, in which is inserted a small machine screw s, which projects outside about 1/8 in. The carrier c, for holding the plate, is made of one piece of tin, with flanges on the sides, the middle being cut out in order to form a spring, which brings the film side of the plate always in the same position as in an ordinary holder. By making the carrier as indicated, economy of space is secured. The smaller size of plates may also readily be exposed by inserting an extra carrier made of tintype, the plate being held in the middle of the field. This method of holding small plates was designed by S. W. Burnham, and is an important feature, as for a good deal of experimental work they answer the purpose of full size plates.
The case is especially adapted for outdoor work, and will probably answer for plates as large as 8 by 10 without any modification.
The carrier e is attached to a rectangular block of wood a, by allowing projections in either end to pass through, when they are bent down and clinched. The block a projects above the mouth of the box 6, forming a light-breaker; an additional security is also obtained by the strip of wood d, which enters the mouth of the box when closed.
For a 4 by 5 plate, the dimensions of the plate holder are as follows: length, 6f in.; width, 4 1/4 in.; thickness, 1/4 in. When the cover 6 is made of tin instead of cardboard, the thickness is only 3/10 in., the other dimensions remaining the same.
One dozen of these cases, holding 4 by 5 plates, are easily carried in one's pocket. To prevent accident, they may be kept securely closed by a rubber band. Burnham, however, has invented a very neat lock, which is entirely automatic in its action, being opened and closed in the act of exposing the plate. It consists of a tongue of spring brass attached to the under side of the cover 6, which enters a pin set in the middle of the block a.
The exposing case (Fig. 241) is a rectangular box, ef, made in 2 parts for convenience, being hinged near the middle so that it can be readily doubled up to render it more portable. It is rigidly held in the position shown in the sketch by means of a long hinge A, on the top of it, and is attached to the camera in the same manner as a double plate holder. In order to secure greater security against the entrance of light, an outside flange is added where it joins the camera. At the left-hand side is a hinged door i, through which the plate holder is inserted.
In the cut a plate holder is shown partially in the case. In the dark chamber f, is inserted a draw rod g, terminating in a mill-head on the outside, and having a cylindrical nut k inside for securing the screw s.
To make an exposure, proceed as follows: -
The door i is opened and a plate holder inserted. When the door is closed, the rod g is pushed in to meet the screws , one or two turns being sufficient to secure it. It is then drawn out as far as it will go; removing the cover b to the chamber f, leaving the carrier c holding the plate in the camera ready for exposure.
The rear end of carrier c is held in position by the block a, accurately fitting the case and the front end is supported by the cover b, which is not entirely removed. After the exposure has been made, the door i is opened, the rod g is unscrewed, and the plate holder is pushed out by pushing in the rod g, which is made long enough for that purpose, '
The cover b is drawn as easily as a slide, and since in the act of drawing it is constantly in a dark chamber, it is not necessary to cover the holder with a cloth, and an exposure may be made in the sun without danger of fog.
The distinctive feature of this method consists, first, in having a compact and light-proof case for carrying the plate; second, in drawing the cover (instead of a slide) in a dark chamber. By this plan all danger of fog in making an exposure is entirely obviated.
The cardboard holders are inexpensive, and it is easier to carry a dozen plates in them than 3 double holders of the usual pattern.
It is adapted for cameras already in use, though in the construction of new ones it may form part of the camera itself; the dark chamber / being hinged for convenience in transportation.
In using this apparatus, it would be more convenient if the ground glass focussing screen could be dispensed with, so as to leave the case constantly attached to the camera daring an expedition. This may be accomplished in various ways.
(1) By fixing the ground glass in the back of the exposing case.
(2) By a movable ground glass in the back of the exposing case, which may readily be brought in the plane of the plate.
(3) By putting the ground glass in a regular carrier and inserting it the same as a plate.
In either case it is necessary to cut a hole in the back of the exposing case and cover it with a door or slide.
Burnham has designed a very satisfactory piece of mechanism for the second method, but it necessarily makes the apparatus more complicated. The third was the suggestion of Prof. H. D. Garrison.