ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



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he sees that now the back combination is fully occupied by a pencil 

 of 76 • 5° ; so that the former pencil of 140^ would, in order to be 

 transmitted, require a much wider back lens. Hence he concludes 

 that abolishing the refraction at the front surface produces loss of 

 aperture with one and the same opening, or necessitates increase of 

 opening for one and the same aperture ; and as the immersion fluid 

 has the same effect as the substitution of the concave surface of 

 admission, the result must also be the same. The wider emergent 

 beams of immersion objectives are therefore shown, it is supposed, not 

 to denote in reality larger apertures ! 



He has, however, fallen into the same mistake in principle as 

 that with the convex hemisphere. When the concave was substituted 

 for the plane front the poiver of the objective was reduced in the ratio 

 of n :1 ; and as the clear opening is not increased, loss of aperture 

 has arisen from loss of power, but not from loss of the refraction in 

 front. As soon as the original power of the objective is restored by 

 deeper curvatures of the posterior lenses, the original opening would 

 be sufficient to transmit the pencil of 140^ air-angle, notwithstanding 

 its greater expansion in the front lens. Thus it is obvious that the 

 anterior refraction cannot account for the smaller openings of dry 

 objectives in comparison with equal power immersion objectives of 

 equal angular aperture. 



The loss of amplification by the concave surface of admission is 

 of course reduced to the fact that the hemisphere 

 amplifies an object at the centre. We obtain the 

 lens h (Fig. 68) with the concave front-surface by 

 cutting out a hemisphere a of the same radius, and 

 as this has previously amplified the object by n 

 diameters, that amount of amplification is lost when 

 it is taken away. 



(c) The Hemisphere as a Condenser. 



There is another phase of the hemisphere puzzle which we can 

 vouch for as having very much puzzled some angular aperturists. 



Comparing the case of an object on an ordinary plane slide 

 (Fig. 69) turned with its under surface to the heavens, and another 

 object on a slide with a portion of a sphere cemented on, the object 

 being at the centre of the sphere (Fig. 70), it is said that the former 

 object receives more light than the latter. The former receives light 

 from the whole 180° of the heavens — that is, all the light from between 



Fig. 68. 



/so° 



the points a and h ; the other object only receives light from between 

 the points c and d, which is a less angular range than a and h. 

 All rays from beyond c and d do not fall upon the sphere, and there- 



