It now only remains to discuss the best mode of mounting the view lens ; which is shewn in the following figure :— The lens is placed in the tube with its concave side to the stop. A ring is then laid against the convex side, and it is screwed tightly in its place. The tube works in a jacket, which is screwed into a flange in the front of the camera ; the open end of it is closed by a cap ; and for better security to the lens a cap is also screwed on at the other end, when the instrument is packed for travelling. Within the tube, a little in front of the lens, is a fixed diaphragm; and in front of that a moveable tube which carries the front dia phragm, which can be so adjusted as to revolve within a projecting box, in order that its full aperture may be presented perpendicularly to the darkest part of the picture, and its edge, so to speak, towards the sky, or lightest objects ; and at the same time any one of a set of stops of different sizes may be introduced as circumstances require. This contrivance is sufficiently explained by the figure. As the tube can be turned in the jacket, the plane of the stop, when placed obliquely to the axis of the lens, can be brought into any required position. The front part of the revolving disc, which carries the stop, has a groove at the bottom and on both sides, and the stop, which is an oblong disc with a round hole in the middle, is slid into this groove like the lid of a box.
It will be seen that an annulus of the circumference of the lens is sacrificed, the lens must therefore be made so much larger as to allow for this. The object of this sacrifice of glass is to prevent the re flection of light from the broad edge of the lens. In the common mode of mounting the view lens this edge forms a broad luminous ring, which is seen on looking into the camera at the open end, and of course the light which enters the eye from this ring would fall upon a sensitive plate if put in the same position.
The diaphragms within the lens-tube entirely prevent the reflec tion of light from the inside of the tube. A strip of black cotton velvet should be wound round the edge of the lens before puttiag it into its cell; and both the inside of the tube and diaphragms should be blackened with a mixture of lamp black and spirit varnish. There are times when it is convenient to• be able to push the diaphragm nearer to the lens, and no mounting is complete which does not ad mit of this being done when necessary.
When a very small stop is used the diameter of the circular space covered by the lens with good detail is nearly equal to its focal length.
Distant objects have a shorter focus than near ones. If f be the principal focal length of the lens, u the distance of an object from it, and v the focal length of the lens for the distance u, then v is determined by the following equation :— 1 1 1 --=-- — v f u Since the field for objects equidistant from the lens is concave and not flat, it follows that the objects at the sides of the picture should, if possible, be nearer to the lens than those in the middle. When this cannot be managed recourse must be had to 8 small atop, which, by cutting off the outer rays of pencils, renders them very long and attenuated in the neighbourhood of their focus, and then the picture may be placed a good deal out of true focus in places without ap preciable loss of detail. By means of the small stop, objects at un suitable and greatly varying distances from the lens, can therefore be brought into tolerably good focus upon a flat focussing screen.