85 



performance of the objective ; but if it still does not 

 come up to the standard, patience must not be lost. 

 The slightest change in the mirror, bull's-eye, or 

 lamp, a touch to the correction collar or micrometer 

 screw is sometimes followed by astonishing results. 

 The beginner should sit down with the expectation 

 that he will fail at the first trial. At each succeeding 

 trial he can easily notice his improvement in manipu- 

 lation and the gain of corresponding results. He 

 should be able to bring the performance of the objec- 

 tive up to the claims made for it, if it has come from 

 the hands of a reliable optician, and should not rest 

 until this is accomplished. 



The writer has often recommended sunlight with 

 generally successful results where ordinary means of 

 illumination have failed. The light is of course 

 intense, and great care will have to be used to modify 

 it by properly using the mirror, but success is often 

 attained and then creates confidence. It is, however, 

 only recommended for this purpose and not for gen- 

 eral use. 



To the histologist it may seem strange that the 

 writer has thus far only spoken of working with 

 objectives on diatoms. This, however, was done 

 advisedly. They are thin, and therefore as suitable 

 as a thin section and far more preferable than a thick 

 one. Their form and structure are easily recogniz- 

 able, and there is very little variation among those of 

 the same kind ; therefore, rules laid down regarding 

 them are generally good. It is conceded by advanced 



