102 THE USE OF THE MICROSCOPE 



Refractive Indices of Ordinary Media 



Air 1.0 Immersion oil 1 . 515 to 1 . 52 



Water 1.33 Clove oil 1.53 



Ethyl alcohol 1 . 36 to 1 . 37 Solid Canada balsam 1 . 54 to 1 . 55 



Acetic acid (glacial) ... 1 . 37 Styrax 1.6 



Paraffin oil 1 . 47 to 1 . 48 Monobromide of 



Xylol 1.49 to 1.50 naphthalin 1.66 



Thin cedar oil 1 . 5 to 1 . 51 Hyrax about 1 . 8 



1. The examination of small objects in air has been 

 to a great extent replaced by balsam mounting, or viewing 

 in immersion oil. But Coles showed (45) that stained 

 spirochaetes were most readily visible on a dark ground 

 when dry, after fixing and staining red; and that the same 

 was true of red-stained bacteria. It may be that it is 

 worth while to apply this method to the filter-passing 

 forms, since a dry objective with as high an aperture as 

 0.95 could be employed, corrected for use without a cover- 

 glass (as for metallurgy). Some of the cocci, nearly 

 invisible by ordinary methods, are, according to Merlin 

 (96), as large as 0.5 micron across. This method might 

 also be useful for demonstrating flagella. It may also seem 

 worth while mounting such objects dry on the under side 

 of a cover-glass of standard thickness, after fixing and 

 staining; and using an oil-immersion objective of 1.4 

 aperture, with a condenser cone of 1.0. Since there is 

 optical contact, the aperture of the objective would not 

 be cut down, and the working aperture would be 1.2. 

 In the writer's experience, this gives sharp images of dry 

 bacteria, stained or unstained. 



2. Since the refractive index of water, 1.33, is different 

 from that of cellulose, and also from that of chromosomes, 

 muscle fibers, etc., investigations in water, sap, serum, 

 normal salt solution, and watery culture fluids, deserve 

 to be carried on, as already stated, as a supplement to the 

 study of balsam mounts. Thin agar jelly with an anti- 

 septic may be used for mounting instead of water. Water 

 of course is especially suited as a mounting medium for 

 objects to be studied by water-immersion objectives, 



