CARE OF THE MfCltUSCUl'E 179 



wiped by a fold of towelling paper or filter paper, without 

 excessive breakage. It has been recommended to wipe 

 covers between two flat blocks of wood covered with 

 chamois leather (Dallinger), but a fresh wiping surface 

 each time is preferable. Well-cleaned covers can be 

 washed in distilled water, and left to dry under shelter from 

 dust. Cleaned covers can also be kept in pure alcohol, 

 and dried by burning off the alcohol. In warm, damp 

 chmates, soft glass covers readily tarnish, and all covers 

 are kept best in xylol. To clean a mounted cover from a 

 drop of cedar oil, the minimum amount of oil should have 

 been used with the objective. This is wiped off with tissue 

 paper towelling or filter paper, followed by paper moistened 

 with alcohol, wet paper, and then dry paper. When using a 

 water-immersion objective, a touch or two with a small 

 roll of filter paper will dry the cover ready for the use of 

 the dry finder objective. 



Care of the Immersion Oil. — This should be kept from 

 oxidation (and consequent excessive thickening and rise 

 of refractive index) by storing it in small bottles well 

 corked and in the dark. The immersion oil by the maker of 

 the objective should be used in each case. The refractive 

 index is 1.515 for the D line, for thickened cedar oil from 

 Juniperus virginiana. On its refractive index and its 

 dispersion, the calculations for the objectives have been 

 made. Some mixtures may show about the same refrac- 

 tive index and dispersion as thickened cedar oil. Anisol 

 is the best single proposed substitute, but it has so different 

 a dispersion as to reduce apochromatic objectives to the 

 level of achromatics (Becher). 



Care of the Slide Bar. — The stage and underside of the 

 slide bar must be kept free from balsam or immersion oil. 

 This is especially important in the Detto slide bar, which 

 is made for circular stages. The stage may be wetted with 

 distilled water, and the slide moved over a layer of w^ater 

 held by capillary attraction. In the Detto slide bar, the 

 moving part is in contact with the stage only at a cork 

 knob, which must be replaced occasionally as it wears out. 



