Sir David Brewster en Neiv Stereoscopes. 21 



it the picture D, as seen by total reflexion from the base BC or 

 be of the prism, figs. 5 and 6, while with his right eye R, fig. 5, 

 he views the same picture directly. The first of these pictures 

 being the reverse of the second D, like all pictures formed by 

 one reflexion, we thus combine two dissimilar pictures into a 

 raised cone, as in the figure, or into a hollow one, if the picture 

 at D is turned round 180°. If we place two diagrams, one like 

 one of those at A, fig. 4, and the other like the other at A, fig. 4, 

 vertically above one another, we shall then see, at the same 

 time, the raised and the hollow cone as produced in the lenti- 

 cular stereoscope by the three diagrams in fig. 4 at C. When 

 the prism is good, the dissimilar image produced by the two 

 refractions at B and C, and the one reflexion at E, is of course 

 more accurate than if it had been drawn by the most skilful 

 artist; and therefore this form of the stereoscope has in this 

 respect an advantage over every other in which two dissimilar 

 figures, executed by art, are necessary. In consequence of the 

 length of the reflected pencil DB + BE + EC + CL being a little 

 greater than the direct pencil of rays DR, the two images com- 

 bined have not exactly the same apparent magnitude ; but the 

 difference is not perceptible to the eye, and a remedy could 

 easily be provided were it required. 



If the conical tube LD is held in the left hand, the left eye 

 must be used ; and if in the right hand, the right eye must be 

 used ; so that the hand may not obstruct the direct vision of the 

 drawing by the eye which does not look through the prism. 

 The cone LD must be turned round slightly in the hand till the 

 line run joining the centre and apex of the figure is parallel to 

 the line joining the two eyes. The same line must be parallel 

 to the plane of reflexion from the prism ; but this parallelism is 

 secured by fixing the prism and the drawing. 



It is scarcely necessary to state, that this stereoscope is appli- 

 cable only to those diagrams and forms where the one image is 

 the reflected picture of the other. 



If we wish to make a microscopic stereoscope of this form, or 

 to magnify the drawings, we have only to cement plano-convex 

 lenses, of the requisite focal length, upon the faces AB, AC of 

 the prism, or, what is simpler still, to use a section of a deeply 

 convex lens ABC, fig. 7, and apply the other half of the lens to 

 the right eye, the face BC having been previously ground flat 

 and polished for the prismatic lens. By using a lens of larger 

 focus for the right eye, we may correct, if required, the imper- 

 fection arising from the difference of paths in the reflected and 

 direct pencils. This difference is so trivial, that it might be 

 corrected by applying to the right eye the central portion of the 

 same lens whose margin is used for the prism. 



