133 



16. Surirella gemma, Ehrbg. 64. to 69. 



17. Cymatopleura elliptica, Breb., 55. to 81. 



18. Navicula crassinervis, Breb. Frus- 



tulia saxonica, Rabh., 78. to 87. 



19. Nitzschia curvula, Sm., 83. to 90. 



20. Amphipleura pellucida, Kg., 92. to 95. 



Whatever opinion one may have in reference to 

 the study of diatoms, the fact cannot be gainsaid 

 that they have been a great aid in the improvement 

 of objectives, are used by opticians to judge their 

 various characteristics and offer a reliable standard 

 for testing the resolving qualities, such as no other 

 objects can. The writer particularly recommends 

 that the test P. angulatum, dry, form a part of 

 every outfit, not so much as a test for resolvability 

 as to use it for study and to acquire quickly a 

 knowledge how to judge the phenomena of spheri- 

 cal aberration. It may be used on powers \ inch 

 and higher and after it has served its purpose may 

 be put on one side to work over objects which 

 come under the particular branch of study which 

 one is following. 



The writer wishes to counteract as much as pos- 

 sible the opinion, which is too prevalent, that the 

 use of diatoms indicates microscopical play and is 

 unworthy of consideration in histological and bio- 

 logical work, but the fact that the optician deems 

 them necessary for determination of optical quali- 

 ties ought at least to indicate that they are a valu- 



