36 THE MICROSCOPE AND ITS KEVELATIONS. 



the ordinary reading distance to the two eyes respectively. This angle, 

 therefore, represents thatf at which the two pictures of an object should 

 be taken in the Photographic Camera, in order to produce the effect of 

 ordinary binocular vision without exaggeration; and it is the one which 

 is adopted by Portrait-photographers, who have found by experience that 

 a smaller angle makes the image formed by the combinations of the 

 pictures appear too flat, whilst a larger angle exaggerates its projection. 

 Now, in applying this principle to the Microscope, we have to treat the 

 two lateral halves (L, R, Fig. 29) of the objective as the two separate 

 lenses of a double portrait-camera; and to consider at what angle each 

 half should be entered by the rays passing through it to form its picture. ' 



To any one acquainted with the principles of Optics, it must be obvious 

 that the picture formed by each half of the objective must be (so to 

 speak) an average or general resultant of the dissimilar pictures formed 

 by its different parts. Thus, if we could divide the lateral halves or semi- 

 lenses L, B, of the objective by vertical lines into the three bands a b c 

 and a' V c', and could stop off the two corresponding bands on either 

 side, so as only to allow the light to pass through the remaining pair, we 

 should find that the two pictures we should receive of the object would 

 yary sensibly, according as they are formed by the bands a a', b b', or c c'. 



1 The -writer has been surprised to find that the advantages of the Stereoscopic 

 Binocular have been treated by certain Microscopists of eminence as altogether 

 chimerical; no real difference (they assert) being discernible between the right-hand 

 and the left-hand pictures. This assertion is obviously placed upon the limita- 

 tion of the use of the instrument to thin transparent objects. It is where the 

 surface is uneven (as is the case with most Opaque objects 1 , or where a Trans- 

 parent object shows different structures in different planes of its thickness (as in 

 injected preparations), that the special value of the Binocular shows itself. The 

 dissimilarity of the^ two pictures of such objects received through the two 

 halves of the objective, was long since demonstrated by Mr. Wenham, who, by 

 covering with a diaphragm, first the right and then the left half of an objective 

 of 2-3ds inch focus and 28 aperture, and carefully drawing the two images thus 

 obtained, found them to be such as would combine stereoscopically, so as to bring 

 out the object in relief. See " Transact, of Microsc. Soc.," N, S., Vol. ii. (1854), 

 p. 1. 



