24 Sir David Brewster on New Stereoscopes. 



a raised pyramid with still less height, and still less symmetry. 

 When the" figure is turned round other 30°, or 90° from its first 

 position, CD will coincide with CD seen directly, and the com- 

 bined figures wdl be perfectly flat. If we continue the rotation 

 through other 30°, CD will coincide with DE, and a slightly 

 hollow, but not very symmetrical pyramid, will be seen. A rota- 

 tion of other 30° will bring CD into coalescence withEF, and we 

 shall see a still more hollow and more symmetrical pyramid. A 

 further rotation of other 30°, making 180° from the commence- 

 ment; will bring CD into union with AF ; and we shall have a 

 perfectly symmetrical hollow pyramid of still greater depth, and 

 the exact counterpart of the raised pyramid which was seen be- 

 fore the rotation of the figure commenced. If the pyramid had 

 been square, the raised would have passed into the hollow pyramid 

 by rotations of 45° each. If it had been rectangular, the change 

 would have been effected by rotations of 90°. If the space 

 between the two circular sections of the cone in fig. 12 had been 

 uniformly shaded, or if lines had been drawn from every degree 

 of the one circle to every corresponding degree in the other, in 

 place of from every 90th degree, as in the figure, the raised cone 

 would have gradually diminished in height by the rotation of 

 the figure till it became flat, after a rotation of 90° ; and by 

 continuing the rotation, it would have become hollow, and gra- 

 dually reached its maximum depth after a revolution of 180°. 



There are two classes of phenomena of a very interesting kind, 

 to which the stereoscope is not properly applicable, namely, 

 those where it is required to unite a great number of similar and 

 equidistant patterns, such as those which compose paper-hangings, 

 carpets, and the openings in the cane bottoms of chairs ; and 

 those in which we binocularly unite, and give a new position to, 

 lines meeting at or converging to a point, the eye being placed 

 at different heights above the plane of the paper, and at different 

 distances from the angular point*. In studying these phseno- 

 mena, we produce the required union by straining the eyes, or 

 by contemplating the objects while the eyes are directed to a 

 point either nearer to or further from them. The power of doing 

 this with facility is possessed by very few persons, and it is there- 

 fore necessary to have a simple and infallible method of effecting 

 the union of such objects without instrumental assistance. The 

 following method, when practised for a short time, will answer 

 this purpose. 



* These two classes of phenomena are described in my paper " On the 

 Knowledge of Distance given by Binocular Vision," published in the Edin- 

 burgh Transactions, vol. xv. p. 663. 



