THE MICROSCOPE AND ITS REVELATIONS. 



being identical; the perspective projection of the object varying with the 

 point of view from which it is seen. Of this the reader may easily con- 

 vince himself, by holding up a thin book in such a position that its back 

 shall be at a moderate distance in front of the nose, and by looking at the 

 book, first with one eye and then with the other; for he will find that the 

 two views he thus obtains are essentially different, so that if he were to 

 represent the book as he actually sees it with each eye, the two pictures 

 would by no means correspond. *Yet on looking at the object with the two 

 eyes conjointly, there is no confusion between the images, nor does the mind 

 dwell on either of them singly; but from the blending of the two a con- 

 ception is gained of a solid projecting body, such as could only be other- 

 wise acquired by the sense of Touch. Now if, instead of looking at the 

 solid object itself, we look with the right and left eyes repectively at pic- 

 tures of the object, corresponding to those which would be formed by it 

 on the retinae of the two eyes if it were placed at a moderate distance in 

 front of them, and these visual pictures are brought into concidence, the 

 same conception of a solid projecting form is generated in the mind, as if 

 the object itself were there. The Stereoscope whether in the forms 



Fio.-in, 



originally devised by Prof. Wheatstone, or in the popular modification 

 long subsequently introduced by Sir D. Brewster simply serves to bring 

 to the two eyes, either by reflection from mirrors, or by refraction through 

 prisms or lenses, the two dissimilar pictures whicn would accurately 

 represent the solid object as seen by the two eyes respectively; these being 

 thrown on the two retinae in the precise positions they would have occu- 

 pied if formed there direct from the solid Object, of which the mental 

 image (if the pictures have been correctly taken) is the precise counter- 

 part. 1 Thus in Fig. 16 the upper pair of pictures (A, B), when combined 

 in the Stereoscope, 8 suggest the idea of a projecting truncated Pyramid, 



'Although it is a comparatively easy matter to draw in outline two different 

 perspective projections of a Geometrical Solid, such as those which are repre- 

 sented in Fig. 16, it would have been quite impossible to delineate landscapes, 

 buildings, figures, etc., with the same precision; and the Stereoscope would never 

 have obtained the appreciation it now enjoys, but for the ready means supplied 

 by Photography of obtaining simultaneous pictures, perfect in their perspective, 

 and truthful in their lights and shades, from two different points of view so 

 selected as to give an effective Stereoscopic combination. 



8 This combination may be made without the Stereoscope, by looking at these 



