250 HOW TO SEE WITH THE MICROSCOPE. 



the use of diatoms in the study of the objectives used 

 under high amplifications, is this : We are enabled to- 

 display on one and the same mount, shells of the same 

 family and species, differing only in size, and we thu^ 

 are on the instant ready to study the work of the ob- 

 jective over each. Now if it so be, and we make it a 

 point that it shall so be that the smaller shell is in all 

 respects the more difficult of the two. Then it occurs- 

 that the student, having mastered such smaller frustule 

 can examine at his leisure the larger ones, and with the 

 certainty that his objective is in at least approximate 

 adjustment; hence he is further prepared to note the 

 difference in the behavior of the object glass over the 

 different diatoms, and thus he arrives at items of the 

 utmost value; all this, too, ^without any change of the 

 mount. 



Still other conveniences there are attending the use 

 of the diatom, their extreme thinness preventing the 

 shadow of one shell from interfering with the defini- 

 tion of another, thus getting rid of a complication 

 which would prove of serious detriment in the early 

 studies of the student, while by and by he can essay ai> 

 attack on the very problem named by merely selecting 

 sueh positions of the amount as contain the little 

 organisms huddled together. Thus learning their char- 

 acter and being thus forewarned, is fore-armed against 

 the time when he shall be brought in contact with 

 other slides presenting the same difficulty, but in a 

 more determined manner. 



But let the reader note this fact. It is one thing to 



