ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 331 



But if it is admitted that tlie illuminating pencil of 140^ in 

 Fig. 72 contains more light than that of 80° in Fig. 71, and that the 

 former must be cut down to 80'' also to establish equality, what 

 becomes of the original argument? If the 140" in the glass of 

 Fig. 72 is more than the 80"^ in the glass of Fig. 71, then the upper 

 pencil of 140° in oil is more than the upper 170° in air. If the 80° 

 in glass below is in both figures equivalent, then the 80° above in oil 

 is equal to the 170° above in air as regards light, and not different, 

 as first alleged. 



(6) Or the angular aperturist will contend that as the immersion 

 objective is illuminated by the pencil of 140° in the slide, the dry 

 objective must be so also. 



First consider the object as not adhering to the slide (see Fig. 73). 



Then he points out that the dry object is at a disadvantage 

 because all of the illuminating 

 pencil outside 82° cannot get out, 

 but is reflected back at the toj) 

 surface of the slide. 



This is of course a wholly in- 



consistent argument. The point mo" ' 



he started with was that it was 



the balsam mounting of the object that prevented the full aperture 



of the dry lens being utilized — the light getting out ; now it is 



the dry mounting that is in fault, and that prevents the light 



getting in. 



Dealing with the point in another way, the object does receive 

 an illuminating pencil of 180°, for that is the extension of the 

 pencil which emerges from the slide. If, according to the angular 

 aperturist, 180° of emission in air is the whole emission, 180° of 

 admission into air is the whole also. If he denies that because it 

 is only 82° in glass, then he admits that an incident cone greater than 

 82° in glass is more than an incident cone of 180° in air; this admits 

 the principle of the unequal equivalent of equal angles in regard to 

 the rays incident upon an object, and there is then no ground for 

 denying it in regard to the emitted cones. 



Secondly consider the object as adhering to the slide. 



Then the object may receive the whole illuminating pencil of 140°. 

 What is the result as to emission, however ? 



If the object is transparent (with a plane surface) no more light 

 than that equal to the reduced pencil of 82° can be emitted into air, 

 whilst the whole 140° can be emitted into oil ; the hemisphere of 

 radiation in air contains, therefore, less light than the hemisphere in 

 oil. 



If the object is structural, the law of diffraction and Fraunhofer's 

 formula show that every incident ray yields several deflected rays, and 

 that in the hemisphere of air there are fewer deflected rays than there 

 are in the hemisphere of oil, i. e. that the whole of the rays emitted 

 into oil is greater than the whole emitted into air. Cf. Figs. 108 

 and 109. 



