312 HOW TO SEE WITH THE MICROSCOPE. 



correction for oblique light ; for observation with cen- 

 tral light the image may be obtained still more colorless 

 if a suitable quantity of a higher dispersive oil (oil of 

 fennel seeds or anise oil) be mixed with that of cedar- 

 wood. But in this case the refraction of the mixture 

 should be reduced to the original refraction of the oil 

 of cedar- wood by adding some pure olive oil ; the test- 

 bottle being always applied for regulating the mixture. 



" A few other oils: The oil of copaiva-balsam and 

 the oil of sandal-wood approach so close to the oil of 

 cedar-wood in refraction and dispersion, that they may 

 be used for it, instead of it, if they should be prefer- 

 able in any respect, provided the test-bottle has stated 

 the right quality of the sample. 



" The use of these oils will not be injurious to prepa- 

 rations which are hermetically closed, as it is likewise 

 necessary for water-immersion, although the black var- 

 nish on some slides (not on all) will be slightly dis- 

 solved by a prolonged exposure to the oil, its action 

 will not be offensive within a moderate time; and in 

 most cases any contact of the fluid with the edge of the 

 covering-glass may be avoided, by the oil not being 

 applied in a greater quantity than is necessary. A 

 minimum drop on the covering-glass, and such a one 

 on the front lens being quite sufficient for observation ; 

 besides that, the varnish of the slides may be perfectly 

 secured against the oil by a solution of shellac in 

 alcohol. 



" A special advantage will result from this mode of 

 immersion for the more difficult dominions of petro- 



