80 



Test-Plate. Almost all microscopists who take 

 an active interest in the capacity of their instruments, 

 supply themselves with a set of test objects, of which 

 P. an^iilatiiin is in most general use, or with a so- 

 called test-plate. These plates consist either of a ser- 

 ies of bands of finely ruled lines ranging from 5,000 

 to the inch to 120,000 to the inch or with a series 

 of diatoms, upon which the markings represent 

 certain divisions of an inch. The one of these 

 which is principally used is made by J. D. Moeller, 

 and consists of a series of 20 diatoms. They are 

 furnished mounted both dry and in balsam, but 

 the latter is the most common. Below is a table giv- 

 ing the numbers, names of the various diatoms and 

 divisions on their surfaces to T y 1 o^ inch A specimen 

 of Eupodiscus Argus begins and ends the series : 



Striae in 

 T oVo of an inch - 



1. Triceratium Favus Ehrbg 3.1 to 4. 



2. Pinnularia nobilis Ehrbg 11.7 to 14. 



3. Navicula Lyra Ehrbg. var 14.5 to 18. 



4. Navicula Lyra Ehrbg 23. to 30.5 



5. Pinularia interrupta Sm. var 25.5 to 29.5 



0. Stauroneis Phoenicenteron Ehrbg... 31. to 30. 5 



7. Grammatophora marina Sm 36. to 39. 



8. Pleurosigma balticum Sm 32. to 37. 



9. Pleurosigma acuminatum (Kg.) Grun 41. to 46.5 



10. Nitzschia Amphioxys Sm 43. to 49. 



1 1. Pleurosigma angulatum, Sm 44. to 49. 



12. Grammatophora oceanica Ehrbg = 



G. subtilissima. . . 60. to 67. 



