150 



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 in adjustable 

 and the tube length in non-adjustable objectives 

 until they are and 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 to the left side and readjust- 

 ing the mirror, observe the effect. It is very 

 probable that the lines will show and if so, con- 

 tinue the advance ; if they do not, give 10 or 20 

 more obliquity and after the structure comes out, 

 again go forward. A point may thus be reached, 

 where with the greatest obliquity which can be given 

 and with the best possible illumination, the objec- 

 tive seems to have come to the limit of its perfor- 

 mance. From the claims which have been made 

 for it, it ought to do better. What is the cause 

 of failure ? Possibly the mirror is not correctly 

 focused, or the adjustment collar may not be cor- 

 rect for oblique light ; perhaps the eyepiece does 

 not give sufficient magnifying power to distinguish 

 the stricz. It may be any one of these causes or all 

 combined. As to the eyepiece, the manipulator 

 must remember the amount of separation of lines 

 in the last object which was resolved and from the 

 gradation in the coarser specimens must judge 



