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pictures are seen in relief by combining the right-eye one with the left-eye one.

The instrument now described is nothing more than a double opera-glass, which itself forms a good stereoscope. Owing, however, to the use of a concave eye-glass, the field of view is very small, and therefore a convex glass, which gives a larger field, is greatly to be preferred.

The little telescopes, Mn, Op, may be made one and a half or even one inch long, and fitted up, either at a fixed or with a variable inclination, in a pyramidal box, like the lenticular stereoscope, and made equally portable. One of these instruments was made for me some years ago by Messrs. Home and Thornthwaite, and I have described it in the North British Review1 as having the properties of a Binocular Cameoscope, and of what has been absurdly called a Pseudoscope, seeing that every inverting eye-piece and every stereoscope is entitled to the very same name.

The little telescope may be made of one piece of glass, convex at each end, or concave at the eye-end if a small field is not objectionable,—the length of the piece of glass, in theirs* case, being equal to the sum, and, in the second case, to the difference of the focal lengths of the virtual lenses at each end.2

7. The Eye-Olass Stereoscope.

As it is impossible to obtain, by the ocular stereoscope, pictures in relief from the beautiful binocular slides which are made in every part of the world for the lenticular stereoscope, it is very desirable to have a portable stereoscope which can be carried safely in our purse, for the purpose of examining stereoscopically all such binocular pictures.

i For 1852, vol. xm. p. 200.

2 These solid telescopes may be made achromatic by cementing concave lenses of flint glass upon each end, or of crown glass if they are made of flint glass.

If placed together with their plane sides in contact, a plano-convex lens, Ab, and a plano-concave one, cd, of the same glass and the same focal length, will resemble a thick watch-glass, and on looking through them, we shall see objects of their natural size and in their proper place; but if we slip the concave lens, c D, to a side, as shewn in Fig. 41,

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we merely displace the image of the object which we view, and the displacement increases till the centre of the concave lens comes to the margin of the convex one. We thus obtain a variable prism, by means of which we can, with the left eye, displace one of the binocular pictures, and lay it upon the other, as seen by the right eye. We may use semi-lenses or quarters of lenses, and we may make them achromatic or nearly so if we desire it. Double convex and double concave lenses may also be used, and the motion of the concave one regulated by a screw. In one which I constantly use, the concave lens slides in a groove over a convex quarter lens.

By employing two of these variable prisms, we have an Universal Stereoscope for uniting pictures of various sizes and at various distances from each other, and the prisms may be placed in a pyramidal box, like the lenticular stereoscope.

8. The Reading-Glass Stereoscope. If we take a reading-glass whose diameter is not less than two inches and three quarters, and look through it with both eyes at a binocular picture in which the right-eye view is on the left hand, and the left-eye view on the right hand, as in the ocular stereoscope, we shall see each picture doubled, and the degree of separation is proportional to the distance of the picture from the eye. If the distance of the binocular pictures from each other is small, the two middle images of the four will be united when their distance from the lens is not very much greater than its focal length. With a reading-glass 41 inches in diameter, with a focal length of two feet, binocular pictures, in which the distance of similar parts is nine inches, are united without any exertion of the eyes at the distance of eight feet. With the same reading-glass, binocular pictures, at the usual distance of 2J inches, will be united at the distance of 2J or even 2^ feet. If we advance the reading-glass when the distance is 2 or 3 feet, the picture in relief will be magnified, but, though the observer may not notice it, the separated images are now kept united by a slight convergency of the optic axes. Although the pictures are placed so far beyond the anterior focus of the lens, they are exceedingly distinct. The distinctness of vision is sufficient, at least to longsighted eyes, when the pictures are placed within 16 or 18 inches of the observer, that is, 6 or 8 inches nearer the eye than the anterior focus of the lens. In this case we can maintain the union of the pictures only when we begin to view them at a distance of 1\ or 3 feet, and then gradually advance the lens within 16 or 18 inches of the pictures. At considerable distances, the pictures are most magnified by advancing the lens while the head of the observer is stationary.

9. The Camera Stereoscope.

The object of this instrument is to unite the transient pictures of groups of persons or landscapes, as delineated in two dissimilar pictures, on the ground glass of a binocular camera. If we attach to the back of the camera a lenticular stereoscope, so that the two pictures on the ground glass occupy the same place as its usual binocular slides, we shall see the group of figures in relief under every change of attitude, position, and expression. The two pictures may be formed in the air, or, more curiously still, upon a wreath of smoke. As the figures are necessarily inverted in the camera, they will remain inverted by the lenticular and every other instrument but the opera-glass stereoscope, which inverts the object By applying it therefore to the camera, we obtain an instrument by which the photographic artist can make experiments, and try the effect which will be produced by his pictures before he takes them. He can thus select the best forms of groups of persons and of landscapes, and thus produce works of great interest and value.

10. The Chromatic Stereoscope.

The chromatic stereoscope is a form of the instrument in which relief or apparent solidity is given to a single figure with different colours delineated upon a plane surface.

If we look with both eyes through a lens L L, Fig. 42, about 2 J inches in diameter or upwards, at any object having colours of different degrees of refrangibility, such as the coloured boundary lines on a map, a red rose among green leaves and on a blue background, or any scarlet object what

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ever upon a violet ground, or in general any two simple colours not of the same degree of refrangibility, the differently coloured parts of the object will appear at different distances from the observer.

Let us suppose the rays to be red and violet, those which differ most in refrangibility. If the red rays radiate from the anterior focus R, or red rays of the lens L l, they will emerge parallel, and enter the eye at m; but the violet rays radiating from the same focus, being more refrangible, will emerge in a state of convergence, as shewn at mv, nv, the red rays being mr, nr. The part of the object, therefore, from which the red rays come, will appear nearer to the observer than the parts from which the violet rays come, and if there are other colours or rays of intermediate refrangibilities, they will appear to come from intermediate distances.

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