MICROSCOPIC OBJECTS. 107 



instruments of different kinds, all equally perfect, the 

 doublet (with Dr. Wollaston's illumination) shows them 

 palest ; next come single lenses and the achromatics, and 

 lastly the reflectors. 



** In selecting a test for comparing the performance of 

 two instruments, it is best to employ one that can be 

 resolved by both of them, otherwise you have no measure 

 of their relative value ; and remember that the illumina- 

 tion is properly conducted with each, otherwise a good 

 microscope may be rejected, and a worse one, by better 

 management, allowed to carry off the palm of victory ; 

 for vdth. difficult objects the illumination requires the 

 utmost tact. I have seen artists with instruments of the 

 very first quaUty spend much time before they could 

 exhibit an object, which at other times they have shown 

 instanter." 



" The following objects I have selected for tests, being 

 easily procured. They are arranged somewhat in the 

 order I have proposed ; but it is difficult to divide them 

 with precision, owing to the diversity of specimens from 

 different subjects of the same kind. 



PENETRATION. 



First Section (Easy). 



Scales of Lepisma saccharina. 



Petrobius maritimus. 



L 2 



