POSITION OF OBSERVER. 253 



The positive convenience resulting from this in the 

 measurement of the focal length of objectives by the 

 method previously described is too obvious to need 

 further comment. 



The series of graduated diatoms by Moller of Wedel, 

 Germany, and generally known as the Moller test 

 Plate, is now to be found in the cabinet of nearly 

 every uiicroscopist, and can be advantageously supple- 

 mented to the list above given. A table showing the 

 mean of the measurement of ten of these plates will 

 be now furnished and thus the plate can on a pinch be 

 made to do approximate duty as a stage micrometer. 



MEAN OF TEN MEASUREMENTS OF MOLLER TEST PLATES,. 

 BY PROF. E. W. MORELEY, M. D., OF HUDSON, OHIO. 



1. Triceratium Favus 3.1to 4. 



2. Pinnularia Nobilis 11.7 to 14. 



3. Navicula Lyra , 14.5 to 18. 



4. Navicula Lyra 23.0 to 30.5. 



5. Pinnularia Interrupta 25.5 to 29.5. 



6. Stauronesis Phoemcenteron 31. to 36.5. 



7. Grammatophora Marina 36. to 39. 



8. Pleurosigma Balticum 32. to 37. 



9. Pleurosigma Acurninatum 41. to 46.5. 



10. Nitzschia Amphoyx 43. to 49. 



11. Pleurosigma Angulatum 44. to 49. 



12. Grammatophora Oceanica=G. Subtilissima 60. to 67. 



13. Surriella Gemma 43. to 54. 



14. Nitzschia Sigmoidea 61. to 64. 



15. Pleurosigma Fasciola 55. to 58. 



16. Surriella Gemma. Longitudinal 64. to 69. 



17. Cymatopleura Elliptica 55. to 81. 



18. Navicula Crassinervis, Frustulia Saxonica 78. to 87. 



19. Nitzschia Curvula 83. to 90. 



20. Amphipleura Pellucida 92. to 95. 



