MICROSCOPIC TECHNIQUE AND REAGENTS. 755 



Greek letter //,), the smaller divisions are equivalent to 10 microns 

 (io/x). For example, suppose, using a low-power objective, 

 that 10 divisions of the ocular scale equal 20 of the smaller 

 divisions of the stage micrometer. Thus, 20 divisions of the stage 

 micrometer are equivalent to 20 times lOju, , or 200 ju; then, since 

 10 divisions of the ocular scale equal 20 divisions of the stage 

 micrometer, one division of the ocular scale is equivalent to i/io 

 of 200 fi, or 20 fi. Or, using the high-power objective, we may 

 suppose that 80 divisions of the ocular scale equal 24 divisions of 

 the stage micrometer. Thus, I division of the ocular micrometer 

 is equivalent to 1/80 of 240^, or 3 /x. Then, if an object has a 

 diameter covering 3 divisions of the ocular micrometer, its diame- 

 ter is equivalent to 3 times 3 p (the value of one division), or 9 /i. 



REAGENTS. The reagents that have been recommended for 

 microscopical work are quite numerous, and, while nearly all of 

 them may have more or less special merit, the number of reagents 

 actually required in practice is fortunately quite small. 



It is important that .the student recognize the necessity for a 

 thorough understanding of the structure of the material under 

 examination rather than place too much dependence upon the 

 effects produced by reagents ; in other words, the study of struc- 

 ture should precede the use of reagents, particularly stains, when 

 it will often be found that the latter can be dispensed with entirely. 



The chemicals that are employed in microscopical work, either 

 as reagents or for other purposes, may be classified as follows: 

 (i) Preservatives, (2) Fixing and Killing Agents, (3) Harden- 

 ing and Dehydrating Agents, (4) Clearing Agents, (5) Stains, 

 and (6) Special Reagents. 



PRESERVATIVES are substances used to preserve material which 

 is to be examined. The most important of these are alcohol ( from 

 40 to 95 per cent.) and formalin [2 to 6 per cent, aqueous or 

 alcoholic (60 per cent, alcohol) solution], the latter of which is 

 considered advantageous in the preservation of specimens contain- 

 ing coloring substances, as leaves, flowers, etc. Almost any anti- 

 septic of the proper strength may be used as a preservative. 



FIXING or KILLING AGENTS are more especially employed in 

 the study of the protoplasmic cell-contents, where by their use 

 the life-processes of the cell are brought to a sudden termination, 



