It is necessary therefore to take two photographs of the object on different plates or films from two different points, these two points must be at the same distance from the object and the camera must in each case point to the same spot. Most stereoscopes are arranged to take two prints each of about 3in. square and the best distance apart of the centres of the plates in the two positions is about 31/4 inches.

Take a large sheet of paper, cardboard, or wood, say 12in. by 12in. (size depends on shape and size of camera) and on it make two points as A and B at a distance of 31/4 inches apart. Join these by a straight line, then find distance of object from camera.

For example, assume this is 8 feet, then from table given below against 8 feet is given an angle of 89 degrees. Set out this angle at both A and B as shown, and produce both ways to edge of paper, etc. Now place the paper from where it is desired to take the photograph and fasten it firmly down, make sure that both lines on paper point to same spot on object. Set camera with centre of the back above point A and centre of the front over the pro duced line. Take the photograph, repeat the operation with a fresh plate from position B. Be careful that the same exposure is given to both and that the paper and object are in the same position and under the same conditions for both exposures.

Before developing, mark which is the right and which is the left hand plate. Print from each and mount with centres 3 inches apart. Be careful that each print is mounted on its correct side. This double print viewed through a stereoscope will give a perfect stereoscopic impression.

To get the best results note very carefully-

1. ;Estimate distance of object to camera correctly.

2. ;Draw the given angle correctly.

3. ;See that lines on paper, etc., both point to same spot on object.

4. ;See that camera is set up centrally with each line in turn.

5. ;See that back of camera is above the two points in turn.

6. ;Be careful to give the same exposure for both photographs.

7. ;Make sure focus of camera is correct.

8. ;Mark each negative right or left.

9. ;Print each to the same density.

10. ;Mount the prints in correct position.

11. ;See that both prints have their centre lines parallel to each other.

It is obvious that the value of cinematograph pictures would be greatly increased if these were also stereoscopic, in fact, it is doubtful if the ordinary cinematograph pictures would then be considered as fit to be compared with them, but at present no commercially possible way has been discovered. It is evident that since stereoscopic pictures differ the right from the left hand, it is quite impossible if both of these were thrown on to one flat screen in the same place to get anything else than a blurr and confusion.

If, on the other hand, the two different pictures be thrown on to two separate parts of the screen, then obviously the centres apart of the two equal the width of one picture at least, and this would compel every viewer to inspect the pictures through some arrangement of lenses, which is not possible commercially on account of the expense etc.

Another suggested method is to give alternately right and left hand pictures on the screen and by a mechanical arrangement permit only the corresponding eyes of the observer to see them, but the expense again prevents this.

Yet another method is to project the two sets of pictures in two different colours on to the screen, and provide each observer with glasses of two different colours through which to view them, but this has the same objections.

From time to time claims are made on behalf of some picture or other that it is a stereoscopic cinematograph, but if these pictures are projected through one lens by white light on to a flat screen and viewed by the eye direct, the claims are sheer nonsense.

It will be seen that as most ordinary objects do not consist of surfaces in one plane, it stands to reason that if a stereoscopic photo be taken of them and the combined result be projected on to a flat screen then it is quite impossible for the two to come to a proper focus on this flat surface, obviously some points will be in focus either in front or behind the screen. This explains the whole difficulty of getting stereoscopic cinematographic displays on a commercial basis, and until a screen can be devised that will adapt itself to the requirements of the picture no real commercial advance can be made. A cinematographic display in which the chief characters disport themselves against a hazy indistinct background does give some of the audience to believe that it is stereoscopic, but it is not so, it merely shows faulty observation.