NOBERTS TEST PLATE. 



119 



difficult, when compared with that into dots. Mr. Wenham, 

 when describing his new illuminator for diatoms, states 

 that he was never successful in the patient manipulation 

 required to resolve this diatom by the old methods of 

 illumination. 



Allusion has already been made to the variations likely 

 to occur in the markings upon the frustules of diatoms 

 and the scales of insects. In order to avoid these irregu- 

 larities, the late Herr Nobert of Pomerania issued a series 

 of test lines ruled upon glass, each band containing lines of 

 a definite number to the inch. The most popular is that 

 known as the 19-band plate, containing lines to the inch as 

 under : 



Herr Nobert often expressed his opinion that the last 

 four bands of this plate would never be resolved by any 

 objective ; but after inspecting Dr. Woodward's photo- 

 graphs of the whole series, he produced another "plate" 

 ruled to the twentieth band, the tenth on which cor- 

 responds to the nineteenth on the old, the twentieth band 

 being ruled at the rate of 200,000 lines to the inch. 



Herr Moller produces what is called a " test-platte," 

 containing 20 diatoms, mounted dry or in balsam; they 

 are arranged upon the slide in a row, at the beginning and 

 end of which is a specimen of Eupodiscus argus. The 



