81 



13. Surirella Gemma Ehrbg 43. to 54. 



14. Nitzschia sigmoidea Sm 61. to 64. 



15. Pleurosigma Fasciola Sm. var 55. to 58. 



16. Surirella Gemma Ehrbg 64. to 69. 



17. Cymatopleura elliptica Breb 55. to 81. 



18. Navicula crassinervis Breb=Frustu- 



lia saxonica Rabh 78. to 87. 



19. Nitzschia curvula Sm 83. to 90. 



20. Amphipleura pellucida Kg 92. to 95. 



It may be said, and perhaps with truth, that a test- 

 plate does not belong to the necessities of an outfit, 

 but considering that it is a guage, on which the opti- 

 cian usually bases the quality of his objectives, it is 

 valuable to the owner of an objective to be able to 

 determine whether, under his manipulation, the objec- 

 tive will perform as well as is claimed for it ; due con- 

 sideration must, however, be given to the fact that 

 there is a certain amount of variation among differ- 

 ent plates, as is shown in the above table. Outside 

 of this, it is a continual incentive to determine the 

 extreme performance of an objective, and it thus 

 becomes the means of acquiring great manipulative 

 skill, which cannot be underrated. The writer is in 

 a position to know that there is great need of this ; 

 innumerable cases have come to his notice where sev- 

 eral objectives of the same kind and equal quality 

 gave unequal results in different hands, and would be 

 highly eulogized by the possessor of one and con- 

 demned by that of another. 



