POSITION OF OBSERVER. 263 



sharp, or whether they are more or less woolley. Com- 

 pare the median line with the upper and lower edges. 

 Manipulate the fine adjustment and notice as to any 

 change of focus necessary as between the edges and the 

 median line; study this median line well, and you will 

 probably detect a shadow from it shading, but only per- 

 haps slightly, the lower entire half of the shell. Grasp 

 the mirror firmly and change its position (not the radial 

 arm) a little in every direction, meanwhile studying the 

 effect of the shadow on the lower half of the frustule. 

 Hunt up other frustules and study them in like man- 

 ner, changing the focus from time to time as may be 

 necessary. 



Selecting an average shell, and placing the same in 

 the centre of the field, and focus sharply. Having ar- 

 ranged the mirror to the best advantage, grasp the collar 

 of the objective with the fingers of the right hand 

 (which now leaves the mirror just as it was), those of 

 the left hand being in contact with the fine adjustment, 

 withdraw the eye for an instant and turn the collar of 

 the adjustment briskly nearly a full half revolution; 

 placing again the eye in position, focus quickly and 

 sharply. Now undertake to decide instantly as to the 

 effects produced. Is there more or less distortion at 

 the ends of the shell than before? How are the edges? 

 Cleaner and sharper, or the reverse? How is that 

 shadow below the median line? Does it shade the en- 

 tire half of the valve more than was the case formerly, 

 or has it consolidated itself to a narrow band under the 

 median line? If you find the least trouble in assuring 



