ORIGINAL COMMUNICATIONS. 



Nobert's Test-plate and Modern Microscopes. 

 By Charles Stodder. 



(From the 'American Naturalist,' April, 1868.) 



Every possessor of a first-class microscope wishes to know 

 what his instrument is capable of doing. To the practical 

 worker it is a matter of much importance, for when the 

 utmost power of his instrument is exhausted he will know 

 that it is a waste of time to endeavour to see more. One of 

 the desirable and important properties of a microscope is the 

 power to show or " resolve" very line lines grouped together, 

 e.g. the striation of the frustules* of the Diatomacese. For 

 the purpose of testing the resolving power of the microscope, 

 the lines ruled on glass by F. A. Nobert, of Barth, Pomera- 

 nia, have long been admitted by experts as the best known 

 test, not only in consequence of their exceeding fineness, but 

 also because they are ruled to a known scale, and because 

 they are so close that physicists have asserted that it is im- 

 possible that they ever can be seen, Nobert himself being in 

 this category ; and all trials of these plates, except those to 

 be herein mentioned, have resulted in failures to resolve the 

 finer lines of these plates. 



The Nobert test is a series of groups of parallel lines ruled 

 on glass thus ||{|||{ |1||||, each succeeding grouj) being finer than 

 the preceding one. Different plates have a different number 

 of groups, ruled to different scales. The one used by Messrs. 

 Sullivant and Wormly (' American Journal of Science,' 

 1861) has thirty bands or groups, the coarsest having its 

 lines T-oVo of ^ Paris line apart, and the finest being -g-oVo > 

 each group or band being about ^ o' o o of ^^ English inch in 

 width, and the whole thirty occupying a space perhaps a 



* A frustule {L.frustrum, a fragment) is one of the fragments into which 

 diatoms separate. 



VOL. VIII. NEW SER. L 



