The Expanding and Folding Camera.—This is merely an expand ing camera, the front and after bodies of which are made to fold. Its merit is, that it can be packed in a smaller compass than the common expanding camera.
These are the principal forms of camera which are found in the hands of the professional photographer ; but a great number of other kinds have been invented by ingenious persons, and are to be found principally among the paraphernalia of eccentric amateurs. Some of these may be briefly mentioned.
Archer's Camera is a box provided with arm-holes and india rubber sleeves in the sides, and a piece of yellow glass on the top. All the manipulation of the collodion process is performed within the camera, and no dark tent is required.
The Bellows Camera is an expanding one, in which the front and after boclies are connected by an expanding cover, resembling , the bellows of an accordeon.
Some other forms of camera have cloth sides connecting the lens with the frame which carries the dark slide, and which blow about in the wind.
Other kinds have been invented, which are remarkable for their extreme portability, being made of a number of hoops of gutta percha, which are inflated, and covered with a cloth. One of these bears the name of the Pocket Camera, and, armed with this inge nious apparatus and a walking-stick tripod, the "pestilent fag and toil," as it has been called, of out-of-door photography, may be avoided. But these instruments are generally found to have a screw loose when required for use.
The bare mention of such contrivances as these will suffice, as they possess no real practical merit, and do not remedy any one of the radical faults of the common apparatus. It is not our object in this work to record and perpetuate matters which experience has shown to possess little or no value, but rather to suggest, if possible, improvements on established forms and methods. Enough, then, has been said about the photographic camera as it is. W e will proceed at once to discuss its weak points, and, if possible, to pro vide a remedy for them.
The radical fault of all the present forms of camera is, that they allow the image to be diluted with light that is extraneous to it, and which gets in and finds its way to the sensitive surface, thereby pro ducing fog on the shadows and general discoloration, and rendering it next to impossible to obtain a clean picture with sufficient exposure to bring out all the details. Now, the way in which dif
fused light enters, and the course it takes from the lens or win dow in front to its destination on the sensitive tablet may be easily exhibited by the aid of a diagram, and the remedy point,ed out.
If the camera is well made and light-tight, the only way in which diffused light can enter it is by the lens. Let us see, then, in what way the construction of the lens is likely to produce this evil.
Fig. 3 represents an ordinary portrait lens mounted in a tube : the anterior and posterior lenses being of the same size, and the inside of the tube being blackened with chloride of platinmn, which merely changes the colour of the brass without destroying its polish. This is the way in which portrait lenses are generally made and sold.
Now observe what follows : Q is the origin of an oblique pencil. Rays from it cover the whole of the anterior surface of the first lens, and the pencil, after refraction, follows the course indicated by the figure. One half of . it is incident on the upper part of the posterior lens, and finds its way to the focus F, but the other half falls on the inside of the tube, and is reflected as shown by the arrows. These reflected rays, together with those from all the other oblique pencils that are similarly cir cumstanced, pass through the posterior lens, and produce a large circular disc of light in the centre of the picture. In this arrange ment, not only has the central part of the picture more light in con sequence of the direct incidence of the central pencils, but one half of the rays of the most oblique pencils is lost, and a portion of those that are reflected from their true course are added to the already too luminous centre of the field.