34 RAMBLES AND REVERIES. 



are brought into view ; and lastly, if the lens is 

 equal to the proper test, the dots start ont into dis- 

 tinctness. It may be as well, however, to remark 

 that the practised microscopist can often bring out 

 dots by manipulating his light and condensers 

 where the tyro would altogether fail. Some frus- 

 tules do not exhibit the dot structure, but appear 

 to be ornamented with ridges. But however highly 

 magnified, the dots where they are shown are 

 all geometrically perfect, and not one of the 

 many thousands is ever found out of its proper 

 place. 



Pleurosigma formosum has been chosen as one of 

 the most useful of these tests, and the appearances 

 which it presents under the quarter inch, one-eighth 

 inch, and one-twelfth inch objectives, may soon 

 become familiar to the student. Another species, 

 P. angulatum, is a good test for still higher powers ; 

 while none but the very best lenses will resolve into 

 dots the markings on Amphipleura pellucida, one 

 of the fresh-water species. There are, of course, many 

 other test objects besides diatoms, but these furnish 

 a very easy and reliable means of examining lenses, 

 and they have the great advantage of permanency 

 in regard to shape and appearance, a vital feature 

 of a reliable standard. 



To mount the valves of Diatoms so as to preserve 

 them they should be put into a solution of sulphuric 

 acid. When the effervescence ceases it is an indi- 

 cation that all organic matter is destroyed. They 

 must then be. washed and treated with nitric acid, 

 and washed again till nothing but dust is left. This 



