FLATNESS OF FIELD. 127 



rnore weight on paper than occurs actually in practice, 

 where the great increase in definition obtained causes 

 the slight deficiency in flatness of field to sink into 

 utter insignificance. Hence, in testing the qualities of 

 an object-glass, the flatness of its field would hardly be 

 called into requisition. 



Nevertheless, flatness of field must have its due 

 weight, and the performance of a first-class objective 

 should not betray any serious error. It will be well, 

 then, when examining an objective, to look after the 

 quality referred to. 



The careful testing of an object glass in this particu- 

 lar, is not such an easy and off-hand matter as might at 

 a glance be presumed; the manipulator may arrive at 

 incorrect results, and thus condemn a glass without 

 due cause therefor, and a word or two as to the proper 

 mode of conducting this test may not be amiss. 



First, it is of the first importance that the object be 

 in itself flat when presented to the objective. Errors 

 may creep in first, because the object is not in itself 

 flat, or, second, because it is mounted on an improper 

 slide, or on the cover thereof very few slides are flat, 

 and covers are notoriously " in wind;" third, the stage 

 of the microscope may not be at right-angles to the 

 optical axis, and fourth, the eye-piece may not suit the 

 objective. To guard as far as possible against these 

 sources of error, proceed thus : 



Select a fine ttage micrometer, having a band of lines 

 just about as close as the objective can well display 

 when viewed with nearly central light, and if you have 



