Sir David Brewster on New Stereoscopes. 23 



5. The Double-Reflecting Stereoscope. 



In this form of the instrument a second reflector is added for 

 the right eye, as shown at M'N', fig. 11, and the effect of this is 

 to exhibit at the same instant the raised and the holloiv cone. 

 The image of B seen by reflexion from MN at the point C is 

 combined with the direct picture of A, seen by the right eye, 

 and forms a hollow cone ; while the image of A seen by reflexion 

 from M'N' at the point C, is combined with the direct picture 

 of B, seen by the left eye. These reflectors may be placed in an 

 elliptical tube, with an opening near the end AB to illuminate 

 the figures A, B, or we may dispense with an opening by having 

 the figures drawn upon thin or transparent paper. When the 

 figures are drawn in transparent lines on a ground of opake 

 varnish, like the diagrams in the magic lantern, the effect is 

 very fine. 



Another form of the double- reflecting stereoscope is shown in 

 fig. 12, which differs from that shown in fig. 11 in the position 

 of the two reflectors, and of the figures to be united. The 

 reflecting faces of the mirrors are turned outwards, their distance 

 being less than the distance between the eyes ; and the effect of 

 this is to unite into a holloiv cone the same figures which the 

 other form in fig. 11 unite into a raised one. The superiority 

 of this position of the reflectors is, that they are more easily en- 

 closed in a tube, and that the instrument is more portable. 



In describing these various forms of the stereoscope, by which 

 the instrument may not only be rendered portable, but may be 

 constructed out of materials which every person possesses, and 

 without the aid of an optician, we have supposed the two dissi- 

 milar figures to be those of the frustum of a cone as seen by 

 each eye separately ; the large circle being the representation of 

 the base of the cone, and the small circle the representation of 

 its truncated summit. If we join similar points of these two 

 circles by lines, as is done in the figures, the conical figure will 

 be more distinct. 



If we take the drawing of a six-sided pyramid as seen by the 

 right eye, as shown in fig. 13, and place it in the total-reflexion 

 stereoscope at D, fig. 5, so that the line MN coincides with mn, and 

 is parallel to the line joining the eyes of the observer, we shall 

 perceive a perfect raised pyramid of a given height, the reflected 

 image of CD, fig. 13, being combined witb AF seen directly. If 

 we now turn the figure round 30°, CD will come into the position 

 AB,aii(] unite with AB, and we shall still perceive a raised pyramid 

 \\it|i less height and less symmetry. If wc turn it round 30° 

 more, CD will be combined with BC, and we shall still perceive 



