TEST-OBJECTS. 193 



of these tests ; and the greater number of them, being objects 

 whose surface is marked by lines, striae, or dots, are tests of re- 

 solving power, and thus of angular aperture only. Hence, as 

 already shown, an objective may show very difficult test-objects, 

 and yet may be very unfit for ordinary use. Moreover, these 

 test-objects are only suitable to object-glasses of very short focus 

 and high magnifying power ; whereas the greater part of the 

 real work of the Microscope is done with objectives of compara- 

 tively low power ; and the enlargement of the angular aperture, 

 which enables even these to resolve (under deep eye-pieces) 

 many objects which were formerly considered adequate tests for 

 higher powers, is by no means an unmixed good. In estimating 

 the value of an object-glass, it should always be considered for 

 what purpose it is intended ; and its merits should be judged of 

 according to the degree in which it fulfils that purpose. We 

 shall therefore consider, what are the attributes proper to the 

 several "powers" of object-glasses, low, medium, and high; 

 and what are the objects by its mode of exhibiting which, it 

 may be fairly judged. 



I. By object-glasses of low power, we may understand any 

 whose focal length is greater than half an inch. The " powers" 

 usually made in this country are of 2 in. or 1J in. focus (these 

 being sometimes made to divide, so as to leave a power of about 

 3 in. focus), 1 in., and 8-10ths or 2-3ds in. ; and they give a range 

 of amplification of from 12 to 60 diameters with the shallower 

 eye-piece, and of from 18 to 90 diameters with the deeper. These 

 are the objectives which are most used in the examination of 

 opaque objects, and of transparent objects of large size and of 

 comparatively coarse texture ; and the qualities most desirable 

 in them, are a sufficiently large aperture to give a bright image, 

 combined with such accurate definition as to give a clear image, 

 with penetrating power sufficient to prevent any moderate in- 

 equalities of surface from seriously interfering with the distinct- 

 ness of the entire picture, and with perfect flatness of the image 

 when the object itself is flat. For the 2 in. or 1J in. objective, 

 no ground of judgment is better, than the manner in which it 

 shows such an "injected" preparation as the interior of a Frog's 

 lung (Fig. 331) or a portion of the villous coat of the Monkey's 

 intestine (Fig. 328) ; for the aperture ought to be sufficient to 

 give a bright image of such objects, by ordinary daylight, with- 

 out the use of a condensing-lens ; the border of every vessel 

 should be clearly defined, without any thickness or blackness of 

 edge; every part of such an object that comes within the field, 

 should be capable of being made out when the focal adjustment 

 is adapted for any other part ; whilst, by making that adjustment 

 a medium one, the whole should be seen without any marked 

 indistinctness. If the aperture be too small, the image will be 

 dark ; if it be too large, details are brought into view (such as 

 the separateness of the particles of the vermilion injection) which 



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