TESTING OBJECTIVES 57 



importance realised by expert workers. Consequently, 

 it is always advisable for a beginner in microscopy to 

 be guided by an experienced friend, or, failing that, 

 to rely upon the maker, whose lenses he has decided 

 to buy, to supply the best for his particular needs, 

 and on no account to purchase lenses that do not bear 

 the name of a well-known house, or old lenses, unless 

 they have been carefully tested. 



Test Objects. There are certain objects which are 

 generally employed as tests, and from the examination 

 of which a skilled observer can judge of the performance 

 of a lens. Photographs are given of a few such on Plates 

 9, 34, 41 and 43, and the points of importance are noted 

 with each. 



The value of such test objects depends not only 

 on the quality of the mount, but more particularly on 

 the observer's familiarity with them. For instance, 

 although certain diatoms or the markings on the scales 

 of Podura are perhaps the favourite test objects for 

 high powers, many observers would use with greater 

 success certain species of bacteria with which they 

 are more familiar. Many of the defects, or otherwise, 

 of objectives may be made out with considerable facility 

 by means of the Abbe test-plate, which is a slide with 

 six silvered cover-glass discs of certain definite thicknesses, 

 or a strip of graduated thickness, cemented on to it. 

 These are silvered on the under side and ruled with 

 fine lines. The lines on the cover-glasses or strip should 

 be examined, and the thickness of cover-glass determined 

 for which the objective has been most accurately cor- 

 rected (p. 69). When this has been found, the lines 

 under it are examined with axial and oblique illumination, 

 and if focussed for axial, should remain in focus, when 

 oblique light is Used, showing that the spherical cor- 

 rection of the lens is satisfactory. Chromatic aberrations, 

 and the nature of the corrections embodied in the ob- 



