82 



Immersion Objectives on Test Plate. To 



determine the highest capacity on test objects, ordi- 

 nary daylight is hardly sufficient ; moderate sunlight 

 or good lamp-light is best suited, but the latter, 

 from the fact that it is always at hand, is preferable. 

 For the purpose of explanation, we will assume that a 

 flat wick lamp and a J, T V or Jg homogeneous immer- 

 sion objective is used. If the right hand is used on 

 the micrometer screw, place the lamp at the right side 

 of the instrument, about 10 inches from it, with the 

 edge of the flame turned toward the mirror. 



The test plate may now be placed upon the stage, 

 and as the diatoms in balsam are very transparent, 

 and therefore very difficult to find, a lower power 

 objective may be used as a finder; bring No. 1, or 

 Tricerdtium Favus, in the center of the field, and after 

 the objective has been removed, attach the immersion 

 objective in the manner prescribed ; the adjustment 

 collar may be placed at zero, as this is about the cor- 

 rect point for standard length of tube. 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. 11, P. angulatum, but observe 

 closely the structure of each one as it comes into the 

 field. Next see whether the objective is corrected ; 

 if the lines and outlines, or middle rib, do not appear 

 to be in one plane, adjust the collar until they are, 

 and then continue the advance toward the higher num- 

 bers until one is reached on which no lines can be 



