42 OF THE CHARACTERISTICS OF THE DIATOMACE.E. 



glass.* The next step is to give an almost imperceptible 

 inclination to the covering glass by gently pressing one of 

 the edges with a fine mounted needle. This causes a tiny 

 stream to flow under the glass, by which means the plants 

 are put in motion, and begin to roll over. Meanwhile the 

 object under examination must not be allowed to escape 

 from the operator's view (a mishap which will inevitably 

 occur if the covering glass be too roughly handled), as it 

 is often most difficult to catch sight of it again. Another 

 reason for exercising great care in the use of the needle is, 

 that in very flat species (such as Himantidium) the hoop 

 is so narrow as to form a mere suture. Consequently the 

 frustule, as it turns over, assumes an erect position for a 

 single second only, immediately resuming its former posi- 

 tion as the stream of water flows back ; so that, unless 

 extreme care be used, the observer will scarcely ever 

 succeed in getting a front view.^ 



It happens occasionally that the plants themselves present 

 both valve and hoop to view, or that they rest against 

 some foreign body in such a way as to show them, without 

 any special manipulation ; but these must be looked upon 

 as happy accidents. 



If it is desired to make a drawing of these delicate 

 organisms, and I would strongly recommend the collector 

 to do so, this maybe accomplished by means of the camera 

 lucida, or Sommering's mirror or steel disk. For a de- 

 scription of these instruments, and the method of using 

 them, I must refer the reader to works more peculiarly de- 

 voted to the microscope. f 



The markings on the silicious valves of the Diatomaceae 



* The thicker this covering glass is (in reason), the more conve- 

 nient is it for the operator ; because very thin glasses are difficult to 

 manage, and the water is apt to overflow their edges, and dim the 

 surface. Of course the positive thickness of the glass must be 

 decided by the powers of the objective in use at the time. 



f Micrographic Dictionary, Introduction, p. xix. ; or Beale's ' How 

 to Work with the Microscope,' p. 20. 



