1899] MICROSCOPICAL JOURNAL. 305 



ed in realgar. The observer should make himself a mas- 

 ter in readily obtaining "critical light," and in illumina- 

 tion generally, for the relative excellence of one glass 

 over another may very largely depend on a fortunate il- 

 luminatiou. Then too he must possess himself with an 

 unbiassed mind, remembering to compare objectives so 

 far as possible of similar N. A., and not to be content 

 with a desultory off-hand examination, but rather that he 

 should examine the performance of each lens step by step 

 as the programme given below proceeds. He must dis- 

 possess himself too of any extravagant expectation, for 

 this may beget a belief that certain performances ought 

 to be present which perhaps may be even theoretically 

 impossible with the aperture employed ; and on finding 

 them absent, such absence of the ideal may induce a 

 strong and unconquerable feeling of disgust that adds a 

 heavy load to be overcome before coming to an unbiassed 

 final judgment. 



Let us take now an inch achromatic and an inch apo- 

 cromatic to compare their performance, using ordinary 

 eyepieces for the former objective and compensating ones 

 for the latter, and use the proboscis of the blow-fly as the 

 test object. The programme suggested — although it is 

 not intended to be exhaustive — may be conveniently ar- 

 ranged as follows : — 



1. — Flatness of field. 



2. — Blackness of shadows in the image. 



3. — Brilliancy of the illumination. 



4. — Resolving power. 



5. — The absence or presence of colored fringes around 

 minute objects such as fine hairs, or dots in diatoms. 



1. Flatness of Field. — By this is meant that after 

 focussing the centre of the object, the edges of the field, 

 if the specimen reaches so far, are equally sharp. The 

 antithesis of flatness of field is called roundness or cur- 

 vature of field, these two terms being in this paper con- 



