256 HOW TO SEE WITH THE MICROSCOPE. 



of surriella gemma is selected for reasons very much, 

 akin to the mention that has been made of podura. 

 These shells are not suited for the purpose of study, 

 but are admirably adapted for comparisons. The 

 actual structure of this diatom is yet a matter sub 

 judice. 



Now let us go back again to the first mounts named 

 in the list. We have had occasion before to assert that 

 one of the advantages resulting from the study of di- 

 atoms is due to the fact that the student in mastering 

 one of the smaller and more difficult ones becomes 

 fortified as to the adjustment of his glass. Supposing 

 then that by some good luck he has succeeded in dis- 

 playing the smaller frustules of the Monmouth or 

 Cherryfield. He can return to the larger shells with 

 tolerable assurance that his glass is in fair correction. 

 But mark this. In order to show the little fellows, 

 there was need of greater obliquity of illumination, 

 which, when return shall be made to the larger shells 

 can profitably be dispensed with. Now although the 

 slide of diatoms offer special conveniences, let the 

 student adopt this line of study, no matter what may 

 be the character of the object under his objective, select- 

 ing the most obscure and diaphanous structure possi- 

 ble to bring into the field, and making sure that there 

 is a sufficiency of oblique illumination. 



From what has been said it may be adduced as a gen- 

 eral rule that it is advisable to study the corrections of 

 the objective by the aid of oblique light, and over the 

 finest structure the mount can be made to exhibit, and 



