13. Surirella gemma, Ehrbg., 43. to 54. 



14. Nitzchia sigmoidea, Sm., - - 61. " 64. 



15. Pleurosigma fascicola, Sm., var.. 55. " 58. 



1 6. Surirella gemma, Ehrbg., - - 64. " 69. 



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



1 8. Navicula crassinervis, Breb., Frustulia 



saxonica. Rabh., - - 78. kt 87. 



19. Nitzchia curvula, Sm., - 83. " 90. 



20. Amphipleura pellucida, Kg., - 92. "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. They are used by opticians to judge the 

 various characteristics of objectives and offer a reliable 

 standard for testing resolving qualities, such as no other 

 object can. The writer particularly recommends that 

 the test Pleurosigma angulatum, dry, form a part of 

 every outfit, not only as a test for resolving power but 

 as an object for study with which to acquire skill in 

 manipulating the microscope. It may be used with 

 powers of 1-4 inch and higher and after it has served 

 its purpose may be put aside for work with objects 

 which come under the particular branch of study which 

 one is following. 



The writer wishes to counteract as much as possible 

 the opinion, which is too prevalent, that the use of 

 diatoms indicates microscopical play and is unworthy 

 of consideration in histological and biological work ; but 



