THE OPTICAL EQUIPMENT 29: 



objective it will be a good lens that, with any ordinary test- 

 object, will stand this test without serious loss of definition. 

 One in which the image remains sharp and clear with an eye- 

 piece magnifying twelve times is, speaking broadly, a good lens. 



With apochromatic objectives this does not apply, for in low 

 and moderate powers these will not break down or show any 

 evidence of deterioration of the image if an eye-piece is used 

 of the highest . available power namely, twenty-seven times. 



With dry lenses of less focal length than one inch, the 

 Podura scale, or any of the coarser diatoms, will answer for 

 purposes of testing. In any case the scale or diatom must 

 be selected so that it has well-defined markings, and these 

 should always show a clean bright image, with no nebulosity 

 or haziness at the edge. What are known as the ' exclamation 

 marks ' on the Podura scale should show up sharply, with 

 their extremities going off to a sharp point. 



For oil-immersion objectives, one of the finer diatoms 

 such as Pleurosigma angulatum, or Amphipleura pellucida is the 

 recognised test ; and certainly for objectives of high aperture 

 either may, in the hands of an expert, fulfil every requirement. 

 If, however, such diatoms are employed, the differences in 

 appearance with varying methods of illumination are so 

 marked that it is impossible for anyone not an experienced 

 microscopist to infer anything from them. The writer is of the 

 opinion that for an ordinary examination of an oil-immersion 

 objective, a stained preparation of bacteria is to be preferred. 

 With this, in conjunction with the method already mentioned 

 for testing the effect of high-power oculars, a great deal can 

 soon be learnt of the qualities of an objective. Preparations 

 should be chosen that are sharply stained, and it is advis- 

 able to select one such as the Bacillus tuberculosis, which 

 has a segmented or beaded appearance ; or some sporing 

 organism such as Bacillus filamentosus since in these it 

 is easier to observe the more subtle differences in the quality 

 of the image. 



For testing for chromatic and spherical correction, as 

 well as for flatness of field, there is no better object with either 

 high or low powers than the Abbe test-plate. This, further, 

 has the advantage that where thickness of cover-glass is of 

 great importance (as it unquestionably is with any high-power 



