66 SELECTION AND USE 



quently, because an objective resolves one specimen of the P. 

 Angulalum, it does not follow that it will resolve all others. One 

 of the most important steps in the direction of uniformity in 

 this respect, at least so far as testing the resolving power of 

 objectives is concerned, is furnished by the test-plate (Probe 

 Platte) of J. D. Moller. Upon a slide of the usual size, he ar- 

 ranges twenty diatoms, carefully selected as to cleanness, and 

 also as to resolvability. Those that he has chosen for the pur- 

 pose are named in the accompanying table. They are arranged 

 on the slide in a line which is about a quarter of an inch in 

 length, the beginning and end of the row being marked by a 

 specimen of Eupodiscus Argus, Ehrbg. The table on the oppo- 

 site page gives the closeness of the lines and the direction of 

 the markings in these diatoms according to the best authorities. 

 In this connection it must not be forgotten, however, that mere 

 closeness is not the only feature which makes a series of lines 

 easy or difficult of resolution. Every micrometer maker knows 

 that of two sets of lines, both ruled at 10,000 to the inch, one 

 may be much more difficult to resolve than the other. The 

 strength of the individual lines has as much to do with it as 

 the mere distance at which they are placed apart. Moller's 

 Probe Platte is furnished of two kinds, dry and in balsam, the 

 latter being, of course, by far the most difficult test. It is an 

 unfortunate fact, however, that even with all the care and skill 

 exercised, even the test-plates of Herr Moller do not always 

 conform to a standard; and, therefore, were it not for the facts 

 just stated, it would seem that the most trustworthy tests are 

 the ruled plates of M. Nobert. 



It is not difficult to test an objective of moderate power for 

 flatness of field, provided we have on hand a suitable object. 

 For this purpose a thin section of wood, or of an echinus spine, 

 is generally chosen. For low powers a very excellent test is 

 one of those micro-photographs which are so common. One 

 showing a sentence or sentences should be chosen in prefer- 

 ence to a picture, since, unless the field of view be flat, the whole 

 of the letters will not be clearly readable at once, while in a 

 picture the effect known as aerial perspective may give rise to an 

 impression of want of flatness of field. In applying tests for 

 flatness of field, it is of course obvious that we must make sure 



