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STEREOSCOPIC PHOTOORAPHY.—The art of stereoscopic photography was practised some years ago, and held public attention for some twenty years or more, after which it gradu ally died away. Many attempts have of late been made to revive it, and it is quite possible that with the new appliances and the better understanding of the theory, it will again receive much attention.

When an artist wishes to make a representation of a solid body, he adopts various means— by the shading and lighting ; in architectural work by geometrical perspective, and in landscape by both these, and also by what is termed aerial perspec tive. Although by this means the effect is often very good, the illusion is far from being complete, as it is rarely possible to mistake the painting for the solid object itself.

By closing one eye, and viewing a painting or photograph with the other, the effect of solidity is considerably in creased. When, however, we see a natural object with both eyes at once, our eyes being about two and a-half inches apart, a little consideration will make it plain that we in reality see two images from slightly different points of view. The right eye sees rather more of the right side, and the left eye rather more of the left side, yet our brain and our vision work together, so that we do not usually see objects double, but single. This is what is termed binocular vision, an effect no single painting or pho tograph can possibly produce.

In the production of stereoscopic photographs we endeavor as far as possible to imitate nature in her provisions for enabling us to see objects with a certain amount of roundness or solidity, and to permit us also to better realise the distances between various objects situated on different planes.

For producing stereoscopic photographs special twin cameras are constructed (see fig 438). In the earliest days, however, a single camera fitted with only one lens was used. An extra base board was provided, upon which the camera was moved from between two and a half to three inches to one side between the first and the second exposure. Although by this means the same effect was, and is still, often produced, yet this method has several drawbacks. For instance, objects in motion and instantaneous effects cannot be made with the single lens. The modern stereoscopic camera is fitted with two lenses side by side, their centers distant about three inches, and their foci equal. These arc generally termed twin lenses. A partition runs down the center of the camera interior to separate the views from the two lenses, and form as it were two distinct cameras.

The usual size of a stereo plate is 61 x 31 inches, but, provided the centers of the mounted prints are not more than three inches apart, the height need not be limited to 31 inches.

Mr. J. Traill Taylor recommends for stereo work a plate 8 x 5 inches. Another well-known practical worker. Mr. W. I. Chad wick, considers the best size of negative plate to be 61 x 41, a standard known as double quarter being half a whole plate, and double the size of a quarter plate.

With regard to the lenses, if required for all-round work, rapid rectilinears will be found the most useful. For landscape work, without straight lines, single lenses give the best effect. They should be mounted carefully on the same horizontal plane. The question of the correct space that should be allowed be tween the two is a disputed one. Our eyes being about 21 inches apart, this is usually the correct distance to get the the natural effect, but for most landscape subjects this can be increased to three inches. The greater the distance between the two lenses the greater the relief ; but this effect can be carried to extremes, producing an unnatural and exaggerated effect. The lenses should be of equal foci, and with perfectly similar diaphragms for each otherwise one picture will be more exposed than the other. Iris diaphragms are too indefinite for this purpose.

A little thoughtful consideration will show that if the two halves of the stereoscopic picture are taken on one plate, these two halves will have to be transposed if the image is required to be seen stereoscopically. They can, of course, be transposed on the plate itself by taking the right, view on the left of the plate and the left on the right, but arrangements for this are rarely made now, the cameras being what are termed " binocular," consisting prac tically of two cameras side by side, making two pictures upon one plate at the same time. With this camera, therefore, if we hold the negative up before us in the same position as it occupied when in the camera, we have the picture taken with the right hand lens on our right hand, and that made with the left lens on our left. But when from this negative we make our positive prints we invert this arrange ment, and the right hand picture is on the left, and, therefore, to get into correct position again, the double print must be cut through the center, and the two pictures divided and transposed. If this be not done, and the pictures viewed thus in the stereoscope, we get an opposite effect to stereoscopic, an effect Which has received the name " pseudoscopic," or false sight.

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