330 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG. 500. 



180 Oil-angle. 

 (1.52 Num. Ap.) 



tive on an ordinary dry-mounted preparation (or on one which is con- 

 nected with the slide, the cover-glass being put on dry), and repeat the 

 observation; you will now see a well-defined circle, a cross section of the 

 emergent pencil, but of less diameter than in the former case, surrounded 

 by a dark annulus, visible by faint diffused light only." 1 The explana- 

 tion of this experiment is, that in focussing an immersion-objective on an 

 object with air above it (i. e., between itself and the cover-glass), the 

 under-surface of the . cover-glass acts as the plane front surface of the 

 system, converting it into a true ' dry ' lens of 180 angular aperture, 

 which gathers-in almost the whole hemisphere of light from the radiant 

 in air- and yet the emergent pencil of rays is much narrower than when 



the same objective is used as an im- 

 mersion, andfocussed on an object in 

 balsam, the extreme divergence of 

 whose rays is not more than 138. 



A wide-angled ' immersion' Ob- 

 jective can therefore utilize rays from 

 an object mounted in a dense medium, 

 such as balsam, which are entirely 

 lost for the image (since they do not 

 exist, physically) when the same 

 object is in air, or is observed through 

 a film of air. And this loss can- 

 not be compensated-for by an in- 

 crease of illumination; because the 

 rays which are lost are different rays, 

 physically, from those obtained by 

 any illumination, however intense, in 

 a medium like air. 



It is by increasing the number of 

 ' diffraction-spectra/ that the rays, 

 admitted from the object contribute 

 to the 6 resolving power ' of the Ob- 

 jective for lined and dotted objects; 

 the truth of the image formed by the 

 recombination of these spectra, being, 

 as formerly shown ( 157), essen- 

 tially dependent upon the augmenta- 

 tion of the number which the objec- 

 tive can be made to receive. 



Upon the ' aperture ' of an objec- 

 tive are dependent (1) its illuminating 

 power, (2) its resolving power, and (3) its penetrating power; the first 

 varying as the square of the numerical aperture, the second being in 

 direct, and the third in inverse proportion to the numerical aperture. 



Whilst Prof. Abbe's investigation has made it clear that the * aper- 

 ture' of an immersion objective may exceed the maximum of that of a 

 dry objective, it is hardly necessary to point out that the act of the excess 

 is a distinct question from that of the 'value of the excess for particular- 

 cases. As the penetrating power of the objective is diminished in pro- 

 portion as the aperture is increased, it is seen that large apertures can 



1 The diameter of the emergent pencil may be accurately measured by the use 

 of an eye-piece Micrometer with the "auxiliary Microscope" of Prof. Abbe's. 



180 Water-angle. 

 U.33 Num. Ap.) 



180 Air-angle. 

 96 Water-angle. 

 82 Oil-angle. 

 1.00 Num. Ap.) 



97 Air-angle. 

 (0.75 Num. Ap.) 



Air-angle. 

 (0.50 Num. Ap.) 



eye-piece 

 Apertometric apparatus, already referred to 



