be given and with the best possible illumination the 

 objective seems to have come to the limit of its per- 

 formance. 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 correct for oblique 

 light ; perhaps the eyepiece does not give sufficient 

 magnifying power to distinguish the striae. 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 spec- 

 imens must judge whether the power is sufficient ; it 

 should be added that for any over No. 14 and under 

 No. 1 8 a i inch eyepiece should be used and for those 

 above No. 18 a power of 3-4 inch for the long 

 tube and of 1-2 inch for the short tube will be 

 necessary. 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 the 

 collar stands and then work it back and forth, watching 

 carefully for results. If this has no influence, return 

 it to its number or to a point where the outline of the 

 object appears most sharp. Now look to the illumina- 

 tion ; vary the distance of the mirror from the object, 

 or if this cannot be done, vary the distance of the lamp 



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