38 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Objectives not exceeding 40° of Angular Aperture, may be shown both 

 theoretically and practically. Taking the average distance between 

 the pupils of the two Eyes as the base of a triangle, and any point 

 of an object placed at the ordinary reading distance as its apex, the 

 vertical angle enclosed between its two sides will be from 12° to 

 15° j which, in other words, is the angle of divergence between the 

 rays proceeding from any point of an object at the ordinary read- 

 ing distance to the two Eyes respectively. This angle, therefore, 

 represents that 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 combination 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. 24) 

 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, r, 

 of the Objective by vertical lines 

 into the three bands a b c and 

 a' b' c', and could stop-off the two 

 corresponding bands on either 

 side, so as only to allow the 

 light to pass through the remain- 

 ing pair, we should find that the 

 two pictures we should receive 

 of the object would vary sensibly, 

 according as they are formed by 

 the bands a a', b b', or c c'. For 

 supposing the pictures taken 

 through the bands b b' to be suf- 

 ficiently dissimilar in their perspective projections, to give, when com- 

 bined in the Microscope, a sufficient but unexaggerated Stereoscopic 

 relief, those taken through the bands a a' on either side of the centre 

 would be no more dissimilar than two portraits taken at a very small 

 angle between the Cameras, and their combination would very 

 inadequately bring out the effect of relief; whilst, on the other 

 hand, the two pictures taken through the extreme lateral bands c c\ 

 would differ as widely as portraits taken at too great an angle of 

 divergence between the Cameras, and their combination would 

 exaggerate the actual relief of the object. Now, in each of the 



