We now proceed to an account of the reflecting stereoscope.
The Reflecting Stereoscope.
Referring to the figure at the commencement of this article. If the visual rays are cut by a plane pq instead of PQ the pictures will be larger than before, and instead of being entirely exterior one to the other, will overlap, and be mixed together so to speak. But if we take the pictures in cameras placed at L and It, with lenses of focal length Lm or Rn, having their axes parallel, and then by means of reflectors throw the virtual images of the pictures so taken into their proper positions on the plane pq, and view these images by eyes at L and 11, a truthful solid image will be produced, as in the former case; because the left eye will not then see the picture from the right station, nor the right eye that from the left station.
The arrangement is exhibited in the following figure :— After what has been said on the subject of the lenticular stereo scope the foregoing figure will only require a few words of expla nation.
The reflectors are placed at right angles to each other, and the pictures at right angles to the dotted line, or base, passing through the angle formed by the reflectors, the distance from that ang-ular point being equal to Lm or Rn, and therefore to the focal length of the lens with which the pictures are taken. The distance of the middle points of the pictures, (that is the point where the axis of the lens cuts them,) viz. in, n, from the dotted base is half the dis tance L R. The pictures are taken in a non-reversing slide, so that their images on the plane pq as seen in the reflectors, are not re versed ; this is an important point to attend to.
The image of the left hand picture ispmb ; that of the right hand pic ture qua, the lines pb, qa, being separated for the sake of clistinctness, but in point of fact the images lie on the same plane. The left eye cannot of course see the image of the right picture, and vice versa, so that the images overlapping produce no confitsion. The image of each picture and the picture itself are symmetrically situated with respect to the reflector by which it is viewed.
All this being understood we come to the mode in which the instrument acts.
A pencil from b in the left picture, after reflexion at the left minor enters the left eye aa ff it had come from the point b on the line pb ; the point b is therefore seen by the left eye in the direction Lb. Similarly, a pencil from b in the right picture, after reflexion at the right mirror enters the right eye 0.1 if it had come from the point b on the line qb ; the point b is therefore seen by the right eye in the direction Rb. These two lines Rb, Lb, are the instantaneous (Erec tions of the optic axes, and being produced they meet at B, which is the true position of the object B. Similarly with respect to the other ' , objects A, C. Therefore by means of the reflecting stereoscope a ' true representation is afforded in natural relief, and actual distance, of the objects in the picture.
The reflecting stereoscope is not open to any single practical or theoretical objection. As an optical instrument it is absolutely perfect, being subject to no defects of distortion or aberration. For any scientific purpose, therefore, the reflecting stereoscope should always be preferred to the other. The reflectors may be made of polished speculum metal if objection be raised to glass mirrors, and the pictures may be taken simultaneously in a camera with double lenses 21 inches from centre to centre, constructed as shown in the following figure, which explains itself.
In this double-reflecting camera the non-reversing slide need not be used.
It now only remains to add a few remarks on taking stereoscopic pictures.
In the first place, the effect of taking the stations wider apart than 2i inches (the parallelism of the cameras being still preserved) is to bring the near objects in the solid image nearer to the spec tator than they were before, and nearer than they are in nature, while the most distant objects remain in their true position. This may be in some cases allowable, because the stereoscope is intended to serve certain educational purposes ; and it may happen sometimes that, by giving bolder relief to objects than they really have, the thing to be explained may be rendered more intelligible.