instrument, about ten inches from it, with the edge of 

 the flame turned toward the mirror. 



Place the test plate upon the stage and, as the 

 diatoms in balsam are very transparent and therefore 

 very difficult to find, a low power objective may be 

 used as a finder ; bring No. i, Triceratium favus, into 

 the center of the field and after the low power objective 

 has been removed attach the immersion objective, 

 which we assume to be a 1-12, in the manner 

 prescribed. Get the best possible illumination with 

 the mirror at the central point and move the test plate 

 from diatom to diatom until it reaches No. n, Pleuro- 

 sigma angulatum, but observe closely the structure of 

 each one as it comes into the field. Next, see whether 

 the objective is spherically corrected. If the lines 

 and outlines, or middle rib, do not appear to be in 

 the same plane, adjust the collar in adjustable or 

 the tube length in non-adjustable objectives until they 

 are, then continue the advance toward the higher 

 numbers until one is reached on which no lines can 

 be seen. Swing the mirror-bar to an obliquity of 

 20 degrees to the left side and, readjusting the mirror, 

 observe the effect. It is very probable that the 

 lines will show and if so, continue the advance ; if 

 they do not, increase the obliquity of the mirror-bar 

 10 degrees or 20 degrees and after the structure 

 comes out, again go forward. A ppint may thus be 

 reached where with the greatest obliquity which can 



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