172 PREPARATION OF BLOOD FOR EXAMINATION. 



information, and a further examination is desired with a higher- 

 power immersion, it will be necessary to attach the cover perma- 

 nently to the slide. 



If blood covers are to be mounted to the best advantage, the 

 first step is to procure a turn-table. The slide is carefully centred 

 on this, and a thin ring of shellac or other suitable cement spun, 

 corresponding to the size of the cover ; a second coat may be 

 applied a few minutes later. A number of slides are prepared in 

 this way, and left for twenty-four hours or more to dry. 



When the slides are ready for use, one is taken and held over 

 a flame for a moment or two to expel all surface moisture and to 

 soften the cement a little. The cover in like manner should be 

 flirted over the flame, to expel all moisture from its surface. It is 

 then applied to the cement ring, care being taken to have contact 

 at all points of the circle. When entirely cold, a fresh ring of 

 cement may be spun around the cover, so as absolutely to seal it 

 at every point. The slide is now ready for examination in any 

 manner, and with any dry or immersion lens. 



It matters not, says the author, whether we are studying the 

 changes in the leucocytes, hunting up the various granules of 

 Ehrlich, or searching for the elusive plasmodia, the optical picture 

 will be vastly superior and much more instructive than any we 

 can obtain in balsam mounts. 



In regard to the substage condensers, he continues, if circum- 

 stances restricted him to the use of a single condenser for all pur- 

 poses, he would choose an achromatic N.A. i, which may be 

 obtained of excellent quality from Zeiss, Bausch and Lomb, 

 Watson of London, and other makers, costing perhaps ten or 

 twelve dollars more than the customary Abbe. With dry lenses, 

 except those of the very widest aperture, it should be used dry — 

 that is, without oil between the condenser and the slide. By so 

 doing the nominal aperture will be impaired about a third and 

 thrown a little off its corrections ; but even then it will be better 

 than any of the Abb^ construction. If used in connection with 

 immersion lenses, oil contact should be used so as to secure the 

 full aperture. If circumstances permit the expenditure, an addi- 

 tional achromatic of N.A. 1*30 to 1*40 should be added ; and for 

 low-power work, an achromatic of low aperture — say^ N.A. o"6o to 



