Sir David Brewster on New Stereoscopes. 19 



It is obvious, from the very nature of the lenticular stereo- 

 scope, that it may be made of any size. The one from which 

 fig. 2 is copied is 8 inches long, and 5 inches at its widest end ; 

 but I have made them only three inches long, and have now 

 before me a microscopic stereoscope, which can be carried in the 

 pocket, and which exhibits all the properties of the instrument 

 to the greatest advantage*. 



If we suppose the two figures at A, fig. 4, to represent a cone, 

 as seen by the right and left eye, the stereoscope will unite them 

 into a raised cone, with the circular apex nearest the eye. If 

 they are placed as at B, they will appear as a hollow cone, the 

 apex being furthest from the eye. In Mr. Wheatstone's stereo- 

 scope, the drawings must be turned upside down, in order that 

 the raised and hollow cone may be seen in succession; but with 

 the lenticular stereoscope, we have only to place three figures, as 

 at C, fig. 4, and between A, B, fig. 2, in order to see at the same 

 time the raised and the hollow cone ; the former being produced 

 by the union of the first with the second, and the latter by the 

 union of the second with the third figures. 



This method of exhibiting at the same time the raised and the 

 hollow solid, enables us to give an ocular and experimental proof 

 of the usual explanation of the cause of the large size of the 

 horizontal moon, of her small size when in the meridian at a 

 considerable altitude, and her intermediate apparent magnitude 

 at an intermediate altitude. As the summit of the raised cone 

 appears to be nearest the eye of the observer, the summit of the 

 hollow cone furthest off, and that of the flat drawing on each 

 side at an intermediate distance, these distances will represent 

 the apparent distance of the moon in the zenith of the elliptical 

 celestial vault, in the horizon, and at an altitude of 45°. The 

 circular summits thus seen are in reality exactly of the same size, 

 and at the same distance from the eye, and are therefore pre- 

 cisely in the same circumstances as the moon in the three posi- 

 tions already mentioned. If we now contemplate them in the 

 stereoscope, we shall see the circular summit of the hollow cone 

 the largest, like the horizontal moon, because it seems at the 

 greatest distance from the eye ; the circular summit of the raised 

 cone the smallest, because it appears at the least distance, like 

 the zenith moon ; and the circular summit of the cones on each 

 of an intermediate size, like the moon at an altitude of 45°, 



* In place of using semilenses, as I at first did, I now use quarters of 

 lenses, which answer the purpose equally well. With a single Jens, there- 

 fore, we can construct two stereoscopes of exactly the same power. This 

 is the first time that a quadrant of a lens has been used in optics. The 

 eye-end of the Btereoscope should consist of two short tubes, with the 

 lenses at their extremities. 



C 2 



