83 



seen. Swing the mirror-bar to an obliquity of 20 

 degrees, 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, give 1 

 degrees or 20 degrees 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 illumi- 

 nation, the objective seems to have come to the limit 

 of its performance. 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 

 focuse'd, or the adjustment collar may not be correct 

 for oblique light ; perhaps the eye-piece does not give 

 sufficient magnifying power to distinguish the striae. 

 It may be any one of these causes or all combined. 

 As to the eye piece, 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 whether the power is 

 sufficient ; it should be added that for any over No. 

 14 and under No. 18 a | inch eye-piece should be 

 used, and for those above No. 1 8 a power of \ inch 

 will probably be necessary, provided a -J- or ^ objec- 

 tive is used. After this condition has been complied 

 with, look to the correction collar of the objective ; 

 to obtain the highest results it very often occurs that 

 a different adjustment is required for oblique light 

 from that for central light. Note the number at 

 which it stands, and then work it back and forth, 



