32 PRACTICAL PHOTO-MICROGRAPHY. 



easier of resolution than any to be found in the cabinet at 

 home, and an optician is likely to choose a pretty even section 

 of an echinus spine as a test for flatness of field. Flatness of 

 field is a highly important quality in an objective for general 

 photo-micrography, and depends chiefly on skilful work on the 

 optician's part. Want of this quality may be hidden by dia- 

 phragms in the ocular, and often is so in badly made apparatus. 

 Finally, it may be said that the tyro microscopist is not 

 capable of making a wise selection of objectives for photo- 

 micrography or any purpose, but an experienced worker will 

 be able to select the good lenses at once in all points except 

 actinic correction, and on that point the optician's word must 

 be taken if trial of the lens is not permitted. The beginner 

 ought therefore to get a skilled friend to choose the lenses for 

 him. "We should consider the following to be a complete 

 battery of objectives, provided they were of first-class con- 

 struction : 3 inch ; 1 inch or 2 inch ; 1 inch ; -J ; J ; -J ; 1 J ^-. 



