46 ELEMENTARY CHEMICAL MICROSCOPY 



ing surfaces) as to converge to an axial point lying just above 

 the plane of the object upon the object slide, it is, of course, 

 essential that the thickness of the object slide be known, for if 

 too thin the illuminating rays will meet too far above the material 

 to be studied, or if too thick the focal point will lie too low; for 

 these reasons optical instrument makers mark upon the devices 

 the object slide thickness to be employed. For example: 



Thickness of object slide. 



Bausch & Lomb paraboloid illuminator 



Zeiss paraboloid condenser 



Reichert reflecting condenser 



Reichert slip-in reflecting condenser 



Leitz reflecting condenser 



Zeiss cardioid condenser for quartz cell 



Spencer Lens Co. Dark-field illuminator. . . . 



1.40 to 1.55 mm. 

 i.o to 1. 10 mm. 

 0.7 to 1. 10 mm. 



2.0 mm. 

 less than i mm. 



1.2 mm. 



1.9 mm. 



Absolutely clean object slides and cover-glasses are essential 

 and great care must be exercised in wiping off the immersion 

 fluid from the condenser to avoid scratching the glass. Lens 

 paper of the highest grade only should be employed, and the 

 wiping off of the fluid should be done with the least pressure 

 possible, otherwise fatty material from the fingers may be forced 

 through the pores of the lens paper upon the glass. A mere trace 

 of grease upon the glass surface will lead to the formation of 

 air bubbles, or will prevent optical contact if water is the immer- 

 sion fluid. Remove all traces of oil with xylene. 



The preparation to be studied must be thin and must be 

 covered with exceptionally clean and very thin cover-glasses. 

 Covering the preparation with a cover-glass is essential. 



In order to expedite the adjustment it is well to have at hand 

 a permanent slide of some material which yields good results 

 with dark-field illumination, as, for example, diatomaceous 

 earth. With such a preparation on the stage the radiant, micro- 

 scope mirror and the condenser are all so mutually arranged 

 as to yield the best illumination of the diatoms; the final adjust- 

 ment is then made by raising of lowering the condenser. The 



