35 



NOBERT'S TEST PLATE, MOLLEE'S DIATOM TYPE 

 SLIDE, AND MODERN MICROSCOPES. 



By Francis Abbott, F.R.A.S., F.R.M.S. 



Nobert's Test Plate, as an instrument for trial, is admira- 

 bly adapted for comparing the quality of object-glasses, sup- 

 plied by different makers,and every microscopist who possesses 

 a first-class instrument naturally desires to know what it is 

 capable of doing ; for when the power and quality of his lenses 

 are known to him, he is convinced it would be only a waste of 

 time to try them further. 



One of the most important tests that the properties of a 

 microscope can be tried by is to clearly " resolve" the very 

 fine lines of the frustules of the Diatomacecd. The value of 

 minute Diatomacecs, is, in their surface markings, which are 

 admirable test objects for the higher power of the microscope. 

 This test was first made known by Messrs. Harrison and 

 SoUitt, of Hull, but unfortunately as a standard measure, a 

 great difference exists in the fineness of the markings in 

 specimens of the same species obtained from different locali- 

 ties. In the Amphipleu^m-jpellucida, the number of strioe 

 range as low as 34, and many specimens present 60, 70, and 

 80 in 1*1000 of an inch. NavicuIcE-rlwmloides, range between 

 30 and 50 in 1*1000 of an inch, so that some specimens may 

 be resolved with a ]-5th, a l-4th, a 4-lOth, or even a 

 half-inch objective, while others require a 1 -8th, or a 1-lOth, or 

 even a higher power. The first of the Diatomacece on which 

 the lines were seen, was the N. IIipi?ocampifs, of Ehrenberg, 

 by Pritchard and Queckett, in the year 1841. In 1844, Mr. 

 Sollitt was in London, and showed the lines on the N. llniata, 

 of Harrison, to the late Mr. Ross, with a 1-8 th of Nachett's ; 

 at that time Mr. Ross could not bring them out with a very 

 fine l-12th he had just finished. This same Diatom jS\ 

 Uniataj has an average of about 60,000 stricB to the inch, 

 which can now be well seen with a good ;f-inch objective. 



For the purpose of testing this resolving power of micro- 

 scopes, F. A. Nobert, of Pomerania, has adopted a method 

 only known to himself, of ruling lines to a known scale of 

 exceeding fineness, so close that physicists have declared it 

 impossible they ever can be seen, and all trials hitherto have 

 resulted in failures to resolve the finer lines of these plates. 



Nobert's test is a series of groups of fine lines ruled parallel 

 on thin glass, such as used for covering, and will admit of the 

 focusing of high powers when reversed and secured on the 

 glass slide ; different plates contain a different number of 



