22 Sir David Brewster on New Stereoscopes. 



3. Tlie Single Prismatic Stereoscope. 



The prismatic stereoscope, represented in fig. 8, consists of a 

 single prism P, with a small refracting angle, capable of refract- 

 ing the image of the figure A, so as just to combine it with the 

 dissimilar figure B, seen directly by the right eye. The second 

 picture should be placed close to A, in order that they may be 

 united by a prism with the smallest refracting angle. There is 

 a slight degree of colour in the refracted image, but it does not 

 injure the general effect. The prism, therefore, should not be 

 made of flint-glass, or any glass with a high dispersive power. 

 A single face ground by a lapidary upon one of the faces of a 

 morsel of plate-glass, the size of the pupil of the eye, will give a 

 prism sufficient for every ordinary purpose. Any person may 

 make one for himself by placing a little bit of window-glass upon 

 another piece inclined to it, and inserting in the angle between 

 them a drop of water. When the figures are small and near one 

 another, a water prism with the requisite angle will scarcely 

 produce any perceptible colour*. 



If we make a double prism, as shown at PP', fig. 9, and apply 

 it to the two dissimilar figures A, B, so that with the left eye L 

 looking through the prism P, we may place the refracted image 

 of B upon A, as seen by the right eye R, we shall see a hollow 

 cone ; and if with the left eye L', looking through the other 

 prism P', we place the refracted image of A upon B, as seen with 

 the right eye R', we shall see a raised cone. 



4. The Singly-Reflecting Stereoscope. 



A very simple stereoscope may be constructed, as in fig. 10, ' 

 by using a small piece of black glass, or plate-glass with one 

 side covered with black wax. This piece of glass MN reflects to 

 the left eye L a reverted image of the figure B, which, when 

 seen in the direction LCA, and combined with the figure A, seen 

 directly by the right eye R, gives a raised cone. The cone will 

 be seen hollow by reversing the figures A, B. As BC + CL is 

 greater than AR, the reflected image of B will be slightly less 

 than A ; but the difference is so little, that it does not affect the 

 appearance of the holloiv or the raised cone. By bringing B a 

 little nearer the reflector MN, the two pictures may be made 

 exactly the same. The small reflector and the dissimilar figures 

 may be fitted up in a conical tube, like that shown in fig. 6, the 

 tube having an elliptical section to accommodate two figures at its 

 further end, the major axis of the ellipse being parallel to the 

 line joining the two eyes. 



* Professor Wheatstone has, we believe, used two achromatic prisms, 

 but thev are not necessarv. 



