90 MICROSCOPY FOR BEGINNERS. 



in the field of his microscope, and to learn its name, if 

 possible, such preparation is unnecessary. 



Diatoms are also peculiar in their structure. In this 

 they have often been compared to a pill-box. The dia- 

 tom is formed of two parts called valves, one of which 

 may be likened to the pill-box proper, and the other to 

 the lid, since it slips over the edge of the lower valve. 

 The entire box-like diatom is called the frustule ; the 

 surfaces of the tipper and lower valves are usually 

 marked and shaped alike, and are called the sides. But 

 when the frustule happens to be turned so that the nar- 

 rowest part, or that part corresponding to the thickness 

 of the pill-box, and called the front, is towards the ob- 

 server, then the shape is so different from that of the 

 valves as to puzzle the beginner. If in doubt about 

 the position, gently tap the cover-glass with a needle, 

 when the frustule will generally roll over on its broad 

 side. This seems somewhat bewildering at first, but 

 there is no difficulty if it is borne in mind that the 

 thickness of the pill-box corresponds to the front of 

 the frustule, and the broad surfaces of the lid and bot- 

 tom to the sides of the valves, 



In addition to the ordinary markings on the valves 

 that is, the transverse lines which are sometimes so 

 coarse that they are called ribs each valve frequently 

 bears a line or narrow smooth band down the middle, 

 while at each end and at the centre there is often a small 

 rounded spot resembling a circular space, but being in 

 reality an elevation, called a nodule. 



