Fig. 197 shows a form of stereoscope that can be used either for viewing the ordinary form of stereoscopic photographs, and also stereoscopic illustration in books, etc. The lens centres are adjustable as to width apart, the lenses being coupled together, so that both move equally about a central position. A flexible black band divides the space between the 2 lenses.

Fig. 197.

Stereoscopic Pictures

Pictures are said to be stereoscopic when they give a solid or lifelike appearance, if viewed or inspected in a certain manner, all objects in the picture then apparently standing out in relief.

To get this effect it is necessary for a start that two pictures or photographs taken of the same object from two slightly different positions be used.

These two pictures are then viewed so that one eye sees only one picture, each eye viewing one picture only, the two different impressions are then combined by the eyes and give a stereoscopic impression to the brain.

Now a stereoscopic camera is merely two duplicate cameras set side by side and arranged so that the lens of each is exposed under the same conditions of time, that is, the shutters are connected together, the two lenses work on to different parts of the same film or plate.

The centre of the lenses apart is generally about 31/4in., this is more than the distance apart of the human eyes, and therefore the stereoscopic effect is slightly emphasised.

Now if a print be taken off tho stereoscopic negative direct, then naturally the picture taken by, say the right hand lens will be on the left hand side of the print, it is therefore necessary to so print that the left hand picture in the negative is on the right hand side of the print and vice versa ; to do this it is of no use whatever to simply reverse or turn over the negative, but one side of the negative must first be printed and then the paper slid along and the other part printed, both in correct position.

The resulting double print or picture is then viewed through a stereoscope, this is made in many forms but essentially it consists of an arrangement holding two lenses, with centres about 21/4in. apart, these lenses are of such a form and arranged in such a manner that when the stereoscopic picture is viewed through them the eyes are able to examine it without strain although the centres apart of the two halves of the print are considerably more than that of the eyes. A partition is generally provided so that both eyes cannot see the same print, though this is not absolutely necessary.

Very beautiful effects are obtained with some subjects, i.e., those with objects at different distances such as woodland scenes, portraits, etc.

It is easily possible to get the stereoscopic impression without the use of the stereoscope, provided the prints are made in a certain size and manner. Now the human eyes have the power of altering the angle between the lines along which each eye views an object, for instance when looking at an object miles away, then the two eyes look along two practically parallel lines, but when looking at, say, a pin head held 12in. away, then obviously the angle between the two different lines of sight must be quite appreciable, also the eyes have the power of altering this angle without effort and retaining it without effort.

It must not be forgotten that no person with one eye only can or ever does get a stereoscopic impression.

The eyes are on an average about 21/2in. apart, therefore if a stereoscopic print be made with the centres of the two halves that distance apart or slightly less, and this be held before the eyes at an easy distance, say 18 inches, then if the eyes be focussed at or for some distant object, it is evident that each eye will act as though viewing one side only, one the right hand and the other the left hand ; this will then give a stereoscopic impression at once.

The print can be made of any height; it is only necessary that the distance apart of the vertical centres should be, say 21/2 inches or less.

When viewed correctly three pictures are apparently seen, the middle one of which is stereoscopic ; there is no injurious effect on the eyes whatever, since they are only doing at will that which they do all day long without effort.

Portraits, etc., taken in this way have a distinctly added value.

Now it is possible to obtain stereoscopic pictures without the use of a stereoscopic camera, by means of any ordinary camera.

As pointed out above, a stereoscopic camera has the two lenses about inches apart, these are both focussed at the same time on the same object, but it must be clearly realised that the two resulting prints are quite different, and that if they were not different it would be quite impossible to get a stereoscopic effect.

Therefore, if two views are taken of an object (when stationary) from two different positions which approximately correspond to the two different positions of the right and left eyes, and the two prints are placed and viewed through a stereoscope in the usual manner, then a perfect stereoscopic result is obtained.

Now clearly the angle between the two axes of the eyes differs with the distance of the object (being observed) from the eyes, therefore it is necessary to arrange that the two prints are taken at different angles depending on the distance of the object from the camera. Since stereoscopic pictures are more pronouncedly lifelike the nearer the object is (in reason) to the camera, the effect is not very great if the object be farther than 100 feet from the camera, this of course depends on the subject. Therefore objects more than 100 feet away can be treated as though at that distance.