28 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



peculiarly adapted to its powers. As the result of this application 

 cannot be rightly understood without some knowledge of one of the 

 fundamental principles of Binocular vision, a brief account of this 

 will be here introduced. — All Vision depends in the first instance 

 on the formation of a picture of the object upon the retina of the 

 Eye, just as the Camera Obscura forms a picture upon the ground 

 glass placed in the focus of its lens. But the two images that are 

 formed by the two Eyes respectively, of any solid object that is 

 placed at no great distance in front of them, are far from 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 convince himself by holding up a thin book in such a posi- 

 tion 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 con- 

 jointly, there is no confusion between the Images, nor does the 

 mind dwell on either of them singly ; but from the union of the 

 two a conception is gained of a solid projecting body, such as could 

 only be otherwise acquired by the sense of Touch. Now if, instead 

 of looking at the solid Object itself, we look with the right 

 and left eyes respectively at 'pictures of the object, corres- 

 ponding 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 

 coincidence, the same conception of a solid projecting form is gene- 

 rated in the mind, as if the object itself were there. The Stereo- 

 scope — whether in the forms originally devised by Professor 

 Wheatstone, or in the popular modification long subsequently intro- 

 duced by Sir D. Brewster — simply serves to bring to the two Eyes, 

 either by reflexion from mirrors, or by refraction through prisms 

 or lenses, the two dissimilar Pictures which would accurately 

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

 throwing these on the two retina? in the precise positions they would 

 have occupied if they had been formed there direct from the solid 

 Object, of which the Mental Image (if the pictures have been 

 correctly taken) is the precise counterpart.* Thus in Fig. 16 the 

 upper pair of pictures, a, b, when combined in the Stereos- 



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

 different perspective projections of a Geometrical Solid, such as those 

 which are represented in Fig. 16, it would have been quite impossible to 

 delineate landscapes, buildings, figures, &c, 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. 



