grating. Abbe's test plate is best. Diatoms are good. 

 No stained object should be used. 



If the spherical correction is perfect (see next para- 

 graph) and one side of a line passing through the center 

 of the field shows a clear, narrow, greenish yellow border, 

 while the other side is fringed with a violet red (second- 

 dary colors) the objective is chromatically corrected. 

 The colors shown in the higher power objectives are of a 

 more primary character, /. e., nearer the yellow and blue. 

 Apochromatic objectives showno color borders in this test. 



The spherical correction of an objective is perfected 

 for a certain thickness of cover glass and a certain tube 

 length, and is influenced greatly by any variation in 

 either. This is especially true with the high power dry 

 objectives. The homogeneous immersion objectives are 

 not sensitive to the variation in the cover thickness 

 because the immersion oil between the cover glass and 

 the lens is of the same refractive index as the glass. 

 They must be used however, with the proper tube 

 length. In testing an objective for its spherical correc- 

 tion it is therefore very important to supply the proper 

 thickness of cover and tube length. It is manifestly 

 unfair to judge an objective on this point without com- 

 plying with these conditions. The test for spherical 

 correction can be made on the same object as used for 

 the chromatic test. If the edges of the lines in the center 

 of the field appear equally sharp and clear when illum- 

 inated by either a narrow central cone of light or a 

 narrow oblique cone without having to change the fine 

 adjustment the objective is spherically corrected. The 

 color remnants mentioned above will be clear and trans- 

 parent, while, if the lens is poorly corrected spherically, 

 these borders will appear muddy and turbid. Defects 

 in spherical corrections can often be corrected by using 

 cover glasses suitable to them, also by changing the tube 

 27 



