The history of three-color photography (1925)

570 History of Three-Color Photography arrangement of Fig. 153, but not with the desired arrangement of Fig. 149. In order to obtain this it is easy to see that the plate must be cut in half, and each plate turned separately on its vertical axis, when the arrangement of the points is correct and the film side of the plate is towards us. The sizes of the plates commonly employed for stereoscopy are 12x16, 9x16 and 10x15 cm., and in the event of plates not being obtainable of this size, it is necessary to use two 9x12 plates or one 13x18; in either case the total breadth is 18 cm. If the finished transparency is to be viewed in an existing stereoscope, the separation of the distance in each picture must be made to suit that apparatus. The most simple way of using an existing stereoscope is by aid of a stereoscopic print, which gives a good effect without fatiguing the eyes. A print should be chosen which contains distant objects, and in which such actually appear distant when viewed stereoscopically. The separation is then measured between two corresponding points of the distance, and may be called a. On the uncut Autochrome a measurement is likewise made of corresponding points in the distance, these corresponding to the separation of the lenses and being called b. These two data enable us to carry out the rule that from the whole breadth of the plate, 18 cm., a portion equal to 180-a-b, must be cut off. In other words, add the separation of distant objects in a good stereo print to that of the similar separation in the Autochrome and subtract the total from the full breadth of the plate, the remainder gives the amount that must be cut off. For example, the separation of a good print was 82 mm., the actual separation of the lenses was 66 mm., consequently the amount to be cut off was 180 — 82 — 66=32 mm., or 16 mm. from each plate. The total picture before trimming is in the condition shown diagrammatically in Fig. 153. If the plate is cut down the middle and the pictures reversed, the separation of the distance will be 180 — 66=114, as shown in Fig. 153. But as the stereoscope requires a distance of 82 mm., a further amount equal to 114 — 8 — 2 must also be cut off, as indicated in the figure. Fig. 154 shows the finished plate. The trimming of the plate may, of course, be done before cutting down the middle, the portions to be trimmed off being situated at either end. From the formula HS-a-b, it follows that the best use is made of the plate the closer the value a-\-b approaches 180. If, therefore, the camera, or the stereoscope, allows of the separation it is an advantage in making full use of the Autochrome plate. Should, however, the total of a and b be greater than 180, there is then no portion to be cut off; the component pictures are placed a little further apart after their positions have been reversed. The grain of the screen-plate, which to the unaided eye, is fairly visible, is often enlarged to an unpleasant degree by the magnifying lenses of the stereoscope, which, when of a focal length of 100 to 150 mm., give about a double enlargement. It is, therefore, advisable as far as possible