Some persons have an idea that two prints from the same negative mounted side by side will give stereoscopic relief, but this is entirely a fallacy and I will explain why. In looking at an object with our two eyes, which are separated normally by a distance of about two and a half inches, we see a little more of the right side of that object with the right eye and a little more of the left side with the left eye, and it is this fact which causes the appearance of the solidity of objects. This may be easily proved by the following simple experiment. Hold one finger up about five or six inches in front of the right eye covering some distant object, at the same time closing the left eye. Now open the left eye and close the right without shifting the finger, and it will be found that the object covered before can now be seen, the finger appearing to have moved away to the right of the object, showing that we get a different view with each eye. Another experiment may be tried as follows: Hold a finger up about nine inches from the nose and look at some distant object, when two indistinct images of the finger will be seen. On directing the vision to the finger apparently two separate views of the distant object previously looked at will appear, one on each side of the finger. This is an easy way of seeing double and without the consequences which sometimes occur when a person sees apparently two keyholes to his door at an early hour in the morning.
About forty years ago almost every house contained a stereoscope and a collection of prints, but at the present day it is comparatively little used, and I am certain there are a number of persons who have never seen a good stereograph. On inquiring at one of the oldest established photographic dealers in London a short time ago for the loan of a stereoscope for a few moments to examine some stereographs I had with me, I was surprised to find they did not possess such a thing in the whole of their stock.
In the chapter on apparatus (Chapter II (Apparatus. The Camera And Plate Holders).) I have already mentioned some of the points for consideration in selecting a camera suitable for stereoscopic work, and in fact the same considerations would equally apply whether the camera was to be used for stereoscopic work exclusively or for general work in addition. If the 1/2-plate or post card size is chosen the same camera will be suitable for either stereoscopic or single pictures, and this is one of the advantages in the selection of one or other of these sizes. With regard to the lenses necessary, it is sometimes possible to pick up a very good pair second hand for a small sum. In stereoscopic work considerable depth of focus is necessary, as a general rule, and therefore the lenses will usually be stopped down to, say, from f11 to f22: and on this account a pair of single lenses will answer the purpose very well if price is an object. They are even preferred by some workers, but as they cannot be used at a larger aperture than f11, they will not be fast enough for some subjects, and a pair of doublets working at f8, or better still f6, are preferable. In the chapter on lenses (Chapter III (The Lens).) I have already referred to the pair of Busch Detective Aplanats with which many of the illustrations in this book were taken. They were of six and a half inches focus, which is quite short enough for Nature work. Long focus lenses have the great advantage of allowing a larger image of the object to be obtained from a certain standpoint than those of short focus, and this is often useful when it is either impossible or inadvisable to approach nearer. I often use two paired combinations of a Zeiss Protar, which are of eleven and a half inches focus and work at f12.5. These give images nearly double the size of a six and a half inch focus lens. When more than one pair of lenses are in use, it is a great convenience to be able to use them interchangeably on the same camera front. This may be done by fitting the flanges of those having the largest diameter on the front of camera or the panel fitting the shutter, and using adapters to carry the smaller diameter threads, which will also fit into the larger flanges.
Another way would be to have a separate panel for each pair of lenses used, which could be fitted to the same shutter. The Thornton-Pickard stereoscopic roller-blind shutter will admit of this being done, and is a very convenient one, as it will allow of either two lenses being used for stereo work or one lens for pictures the full size of the plate, and in choosing the camera it should be ascertained that the front of it will permit of a shutter of this type being fitted. The shutter mentioned above can be had with an arrangement for separating the lenses, which is sometimes very useful in practice. For post card size it should be possible to alter the distance between the optical centres of the two lenses from about two inches or two and a quarter inches to three inches. As many of the subjects taken will be at fairly close quarters, I find in my own practice that a separation of from two and a quarter to about two and a half inches is used most often. In the case of our eyes the rays converge at a greater angle from a near object than from a distant one, but as we can accommodate our vision to the altered conditions we do not get an effect of exaggerated relief. With the lenses, however, the case is different, and to avoid this exaggerated effect of relief we must alter the separation of the lenses, using them nearer together for near objects and at a greater separation for distant objects. I have found for Nature work that it is not necessary to separate them more than two and three-quarter inches, although for very near objects two inches separation is sufficient, if it can be obtained. If a fixed separation is a necessity, then let it be two and a half or two and a quarter inches. A level should be fitted to the camera, one of the double patterns being best, as in taking subjects which include horizontal lines, it is important to see that they are represented horizontally in each picture on the focussing screen.