When the slide is filled and closed, and when the remaining plates have been again carefully wrapped up or have been placed in a light-tight box, the photo grapher may issue from the dark room. The dark slide should be placed under the coat, or may be wrapped up in a piece of black cloth. It is not advisable to leave it for any length of time in bright light, as the smallest fault would result in the destruction of one or both plates.
A subject must be chosen to make an experimental exposure on. The best thing is a brightly-lighted land scape, which should show as broad contrasts of light and shade as possible. The composition of the scene as a picture need not be considered at this stage. The camera is placed so that the subject will be lighted from the side,—that is to say, the position is such that the sun is neither before nor behind the camera.
The lens is fixed in position and the operation of focussing commences. As we said, the lens may be of any of the different varieties which are used. It is best, if possible, however, that it be not a portrait lens. If the lens have movable diaphragms or stops, they must be removed, so that the aperture or opening is the largest which the lens is constructed to work with.
On covering the head with the focussing cloth, so that the focussing screen or ground glass is in comparative darkness, and looking at this latter, an inverted image more or less distinct of the object opposite the camera will be seen. Now we work the adjustment which alters the difference between the lens and the ground glass, and which generally is either a screw or a rack and pinion. We try first one direction and then another till we find the image getting sharp. We then look at some bold object near the centre of the ground glass, and adjust till this appears quite sharp. We have now, in those lenses which are fitted with diaphragms or stops, to select a stop to insert in the slit of the lens. These are generally small plates of thin blackened brass, with various-sized holes in the centre of them. The plates may, any one of them, be slipped into a slit in the brass work of the lens, which is made for their reception.
In the case of some lenses there is, in place of the set of separate stops or diaphragms, a disk, which is an attach ment to the brasswork of the lens, but which can revolve so as to let any one of several different-sized holes in it take the desired central position. The use of the diaphragms will be fully explained in the chapter on photographic optics. Meantime, for the preliminary experiment, it will be best to use the smallest stop with which the lens is supplied. When this is adjusted we are ready to expose.
The cap is placed on the lens, the ground glass is removed or hinged up according to the construction of the camera, and the dark slide is put into its place. This latter operation is best done under cover of the focussing cloth, which should remain over the whole of the camera with the exception of the lens till the exposure is complete.
The shutter or sliding part of the dark slide is withdrawn, when it will be understood that nothing prevents the image from falling on the sensitive film except the lens cap. This latter is now gently removed. If the subject be a brightly-lighted landscape it may be kept off for two or three seconds, when it is replaced. The shutter of the slide is replaced. It will be best to make another exposure of longer duration on the other plate before the camera is shifted or the stop removed. A comparison of the two resulting negatives afterwards will be instructive. We may then, im mediately after we have replaced the shutter, take out the dark slide, reverse it, replace it, withdraw the other shutter and expose once more, giving this time, say, ten seconds. The times may be counted from the seconds hand of a watch, or if the photographer be tolerably good at counting seconds he may use that method. It is quite accurate enough.
After exposure the shutter is once more wrapped up or placed under the operator's coat, and the process of development may be performed.