THE OPTICAL EQUIPMENT 27 



both optically and mechanically easier to produce a lens of 

 high aperture and proportionately high magnifying power 

 than the reverse. The consequence is that a lens which, for 

 instance, might be described as a one-inch objective, with an 

 aperture that would be a high one for such a focal length, is in 

 some cases little more than, say, three-quarters of an inch. 

 This is shown by Fig. 8, where a was photographed with a 

 one-sixth-inch achromatic objective, and b with a 4-mm. 

 apochromat ; it will be seen that the achromatic lens gives a 

 distinctly higher magnification although the focal lengths are 

 practically the same. 



Apochromats are generally described with more exactness, 

 as their manufacture was for some time confined to one 

 firm, and consequently they have been more carefully 

 standardised. 



If it is desired to know the exact magnification of an 

 object, either visually or photographically, it will be necessary 

 to determine it by the aid of a micrometer. The methods 

 to be employed for such determinations will be fully described 

 later. (See Chap. VIII, p. 153.) 



Keference should here be made to some new objectives 

 which have been recently produced by Messrs. Zeiss, to 

 computations by Dr. von Kohr, and termed by them ' mono- 

 chromats.' These are constructed for photography only, and, 

 as their name indicates, they are only designed for use with 

 monochromatic light of a definite wave-length, since they are 

 not chromatically corrected. To make them as far-reaching 

 as possible in their optical efficiency they are specially 

 designed for use with ultra-violet light of short wave-length. 

 (See Chap. XI, p. 250.) 



For exhaustive methods of testing and choosing objectives, 

 the reader must refer to books treating of microscopy purely, 1 

 but it may be convenient if some simple tests are indicated here, 

 so that in purchasing lenses reliance will not be entirely placed 

 on the seller, and a serious worker will derive some extra satis- 

 faction from discerning the differences in the lenses for himself. 

 It is advisable, however, to purchase only from makers of high 

 repute, and in that case no difficulty will be encountered in 



1 See Carpenter's Microscope and its Revelations ( J. & A. Churchill, London) 

 or Spitta's Microscopy (John Murray, London), 



