In the next place, the effect of directing the axes of the lenses to the same point at a finite distance introduces distortion in the solid image, unless the pictures are placed in the stereoscope at the same angle of inclination to one another as the focussing screens of the cameras. When, however, the stations are taken very wide apart (several feet, for instance), if the parallelism of the cameras i)e preserved, the pictures are partly thrown outside the focussing 'Teen, besides being taken with very oblique pencils ; so that, hen correct principles of operation are once departed from, for any arpose, other changes in the arrangement of things become neces xy. We cannot, however, within the compass of the present work, more than explain completely the theory of the correct form of the instrument ; it would require a separate treatise to follow up the subject through all the modifications which it may assume. Enough has been said to enable any intelligent reader to think out the remainder for himself.
In printing stereoscopic pictures from a negative, taken in a double-lens camera, by contact in the pressure frame, it must be borne in mind that the print requires to be cut in half and the pic tures transposed, in order to bring the picture that was taken from the left station before the left eye in the stereoscope, and vice vend. If this be not attended to, a pseudoscopic effect is produced.
In printing stereoscopic transparencies by means of a lens, the following plan may be adopted :— A copying camera, rather more than double the length of the stereoscopic camera, is provided, and the lenses of the stereoscope are fixed in the middle of it ; there must also be a partition divid ing the camera in half lengthways. In this way the left lens copies the left picture at the same-time that the right lens copies the right picture. The negative must be placed with its back next to the lenses at one end of the box, and the sensitive positive plate in a common slide at the other end. The camera is then directed towards the sky, and the wet collodion process employed. An exposure of a few seconds is sufficient. 'The positive need not be divided and the pictures transposed, for, when placed in the stereo scope with its plane side next to the lenses, and a ground glass laid itosinst the film, the pictures are in their right position to be viewed.
Eby putting the lenses midway between the negative and positive, the positive becomes of the same size as the negative. The best lenses to employ are portrait lenses, with a small stop between the back and front lenses in each.
In the stereoscopes and stereoscopic pictures commonly sold there are the followmg serious defects :— 1st. The pictures are frequently taken in converging cameras, and then mounted upon the same fiat surface.
2nd. The pictures are generally mounted so wide apart that the most distant objects in each are wider apart than the distance between the centres of the eyes.
3rd. An attempt is made to obviate the evil produced by the above practice, by using semi lenses in the stereoscope which dis place the images. This of necessity produces clistortion, because straight lines are always represented by curves when the outside Fut of a lens is used to view objects through, instead of the centre.
4th. The displacement of the images is in general so great as to cause the optic axes to converge to points situated within two or three feet from the nose, instead of the true distance of the objects. The effect of this is to make the solid picture look like a small model of the object, which the spectator could, if he chose, lay his hand upon, or touch with a yard measure.
Lastly. The focal length of the lenses of the stereoscope is in general 6 inches, while that of the lenses in the camera is only 4 inches. This makes objects appear much smaller than they do in nature.
To sum up. The common stereoscope and pictures make objects look very near, very small, and distorted. The stereoscope described and recommended in the present article makes them look of their true size, at their true distance, and without perceptible distortion.
In this article the term " solid image" has been several times used. The employment of this term may perhaps be thought ob jectionable ; we do not, however, know of a better, and if the thi g meant has been clearly understood, the end has been answered.