CHAPTER X 

 THE OBJECT 



Stained Objects in Balsam, or Immersion Oil. — Many 

 or most of the objects usually examined have been fixed, 

 stained, and permeated with a medium of higher refraction, 

 such as Canada balsam in xylol, or immersion oil (as in 

 most smears of bacteria, and blood corpuscles). The 

 fixing and staining are, of course, necessary, and the artifi- 

 cial colors show better in balsam or cedar oil, since the 

 differences of refraction are lessened or even annulled. 

 But too exclusive a dependence need not be placed on 

 observances of color differences, to the exclusion of refrac- 

 tive differences, especially when the stain is not differential. 

 The excellent modern water-immersion objectives should 

 not be neglected. For by a water-immersion objective 

 alone can perfect views be readily obtained of the fine 

 structure of plant or animal preparations in serum, sap, 

 or any watery medium, that is, in their natural state, if 

 they are of a certain depth, about 0.01 milUmeter or more, 

 beyond which high-power oil-immersion objectives give 

 fogged images. Differential staining in one or two colors, 

 however, has been the chief help in some branches of 

 microscopical work. 



Objects in Watery Media. — The use of stained speci- 

 mens, smears, or sections, in balsam or immersion oil, 

 should probably be regarded as supplementing, or being 

 supplemented by, the study of living or fixed objects in 

 water, serum, sugar solution, acetic acid, or similar liquid 

 of low refrangibility. In specimens of chromosomes fixed 

 in 45 per cent acetic acid, and simultaneously stained with 

 iron carmine, the contours are sharply defined; whereas, in 

 stained chromosomes (or bacteria) in balsam or immersion 

 oil, the contours are usually indicated wholly, or almost 

 wholly, by the cessation of stain. Hence, it is well to study 



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