20 Sir David Brewster on New Stereoscopes. 



because their distance from the eye is intermediate. In the 

 accompanying model this effect will be distinctly seen, by placing 

 three small wafers of the same size and colour on the square 

 summits of the drawings of the cones or four-sided pyramids. 

 No change is produced in the apparent magnitude of these 

 circles by making one or more of them less bright than the rest, 

 and hence we see the incorrectness of the explanation of the size 

 of the horizontal moon, as given by Dr. Berkeley*. 



When the observer fails to see the object in relief from the 

 cause already mentioned, but sees only the tiro drawings, if there 

 are two, or the three drawings, if there are three, the plane of 

 the drawings appears deeply hoJIoiv ; and, what is very remark- 

 able, if we "look with the eccentric lenses at a flat table from 

 above, it also appears deeply hollow j and if we touch it with 

 the palm of our hand, it is felt as hollow, while we are looking 

 at it, but the sensation of hollowness disappears upon shutting 

 our eyes. The sense of sight, therefore, instead of being the 

 pupil of the sense of touch, as Berkeley and others have believed, 

 is in this, as in other cases, its teacher and its guide t- 



2. Tlie Total-Reflexion Telescope. 



This form of the stereoscope is a very interesting one, and 

 possesses valuable properties. It requires only a small prism 

 and one diagram, or picture of the solid, as seen by one eye ; the 

 other diagram, or picture which is to be combined with it, being 

 created by total reflexion from the base of the prism. This in- 

 strument is shown in fig. 5, where D is the picture of a cone as 

 seen by the left eye L, and ABC a prism, whose base BC is so 

 large, that when the eye is placed close to it, it may see, by 

 reflexion, the whole of the diagram D. The angles ABC, ACB 

 must be equal, but may be of any magnitude. Great accuracy 

 in the equality of the angles is not necessary ; and a prism con- 

 structed by a lapidary out of a fragment of thick plate-glass, the 

 face BC being one of the surfaces of the plate, will answer the 

 purpose X- When the prism is placed at abc, fig. 6, at one end 

 of a conicai tube LD, and the diagram D, at the other end, in a 

 cap which can be turned round so as to have the line mn, which 

 passes through the centre of the base and summit of the cone 

 parallel to the line joining the two eyes, the instrument is ready 

 for use. The observer places his left eye at L, and views with 



* Berkeley's Works, p. 98; Essay on the Theory of Vision, § 67-78. 

 Lond. 1837. 



t See Edinburgh Transactions, vol. xv. p. 672. 



% In this case the prism may have the form BctfC, fig, 5, the parallel sides 

 BC, cd being the original faces of the piece of the plate-glass, and the in- 

 clined faces Be, Cd only, the work of the lapidary. 



