ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
247 
Abbe’s Apertometer.* — This apparatus, for determining the nume- 
rical and angular aperture of objectives, f is seen in its latest form in 
% 51. 
Fig. 51. 
Hartnack’s Illuminating Apparatus for Monochromatic Light, f — 
In this apparatus, shown in fig. 52, the light from the slit Sp is rendered 
Fig. 52. 
parallel by the collimator C, and after passing through the pair of 
prisms P x P 2 is projected upon the object by the objective O. The 
different colours of the spectrum can be made to successively traverse 
* Zeiss’ Catalogue, No. 30, 1895, p. 29. 
+ See this Journal, 1878, p. 19, and 1880, p. 20. 
X Zeiss’ Catalogue, No. 30, 1895, pp. 66-7. 
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