94 SELECTION AND USE 



that the test used is itself flat. Common glass slides are not 

 flat, and as they are used for the cheaper kinds of micro- 

 photographs, this fact may give rise to errors if we are not care- 

 ful in our selections. The best slides are cat from glass plate 

 which has been ground and polished so that the sides are per- 

 fect planes, and it is this kind only that should be used. Care 

 should also be taken to see that the object lies flat on the slide, 

 and is not distorted by the cover. We have seen an objective 

 condemned because it did not show all the diatoms in the field 

 of view in focus at once, when the fact was that the diatoms 

 were attached to t'le cover which was slightly wavy as cov. rs 

 often are. When it is suspected that the fault is not in the objec- 

 tive, but in the slide or cover, the object should be carefully 

 passed across the field of view, and the changes in focus noticed. 

 This will in general tell where the defect lies, for if the part 

 that is apparently foggy should move as the object nlbves, it 

 shows that the object itself is not flat. It has been 'recom- 

 mended by high authority to test objectives for fl itness of field 

 by strewing some fine powder on a slide and seeing whether all 

 the grains are in focus at once. For obvious reasons this is a 

 very unreliable method. 



Penetration in low powers is perhapa most readily deter- 

 mined by the examination of opaque objects of considerable 

 thickness. The round pollen grains of the hollyhock, and the 

 rounded forms of the polycystina are excellent tests for objec- 

 tives of an inch or inch and a half. Lower powers ought t > 

 show coarser objects in all their dimensions, while for those of 

 medium power the coarser cellular tissue of plants answers very 

 well. It is more difficult to indicate a good test for penetra- 

 tion in the higher pjvvers, in which, by the way, wo have often 

 seen want of penetration mistaken for want of flatness of field. 

 This arose simply from the fact that scarcely any object is 

 absolutely flat, and hence, as explained under another head, 

 the curvature of the object is sometimes taken as an indica- 

 tion of a defect in the objective. Want of good working 

 distance makes itself obvious during the examination of any 

 object suited to the objective. 



