68 MICROSCOPY FOR BEGINNERS. 



surface sculpturings, the delicacy and the closeness of 

 which defy description. So fine and close together are 

 the surface lines of some that they are used to test the 

 good qualities of the best and highest power objectives: 

 There are no perfectly smooth diatoms, although many 

 may appear so to a low-power lens ; but the splendid 

 glasses of the best American makers will compel any 

 diatom to show just how it is marked and roughened. 



In each end of many desmids, especially in the cres- 

 cent-shaped ones, is a small colorless, apparently circu- 

 lar space containing numerous very minute black parti- 

 cles in incessant motion. These little granules, which 

 are said to be crystals, are sometimes so few that they 

 can be counted if sufficiently magnified, while in other 

 individuals they are innumerable. Their motion resem- 

 bles the swarming of microscopic bees. It can scarcely 

 be described, but once seen it can never be forgotten. 

 The spaces containing them are called vacuoles, and are 

 never present in diatoms. It is true that in some of 

 the latter, when dying or dead, many minute black par- 

 ticles are visible, dancing and swarming in clusters with- 

 in the cells, but this is common to many microscopic 

 creatures after death. In the desmids there is also often 

 seen a circulation of the protoplasm similar to the cy- 

 closis in the leaf-cells of Anacharis, a movement of the 

 cell contents never observed, so far as I am aware, in 

 any diatom. Between the cell-wall and the green col- 

 oring matter, the chlorophyl, there seems to be a nar- 

 row space filled with colorless protoplasm, and it is here 



