18 ELEMENTARY CHEMICAL MICROSCOPY 



copist cannot hope in practical work to carry the resolving power 

 of his instrument beyond this limit. 



In ordinary work a magnification of from 7 50 to 900 diameters 

 is the upper limit of true usefulness in the study of details of 

 structure. Above this point the worker must be an exceptionally 

 keen and skillful observer in order that he may properly interpret 

 the appearances seen in the images formed. 



It is best, therefore, to make it a rule to work with low magni- 

 fications. 



Study the preparation thoroughly and have recourse to high 

 powers only when absolutely necessary. The dangers of errors 

 of interpretation are thus greatly reduced and fatigue and eye 

 strain practically eliminated. Moreover, it must never be for- 

 gotten that with high-power objectives a very small area only 

 is visible and the relation of the structure of the tiny area in 

 the field of view to that of the adjacent areas bounding it may 

 often be overlooked or be only imperfectly understood. Faulty 

 deductions^ are apt to follow. 



The student will find that when using a Bausch & Lomb 

 chemical microscope and medium power eyepiece (7.5 X) the 

 diameter of the circular area visible with a 32 mm. objective 

 is approximately 4 mm.; with a 16 mm. objective the visible 

 area is reduced to a circle about 1.9 mm. in diameter; with an 

 8 mm. objective the tiny circular area is only 0.68 mm. in diam- 

 eter, while with a 1.9 oil immersion the circular area visible 

 is less than 0.2 mm. in diameter. 



Use low-power eyepieces whenever possible. In the study of 

 objects mounted between object slides and cover- glasses, obtain 

 increased magnification by higher powered objectives, but with 

 uncovered objects, drops of liquid and in microscopic chemical 

 analysis it is best to obtain increased magnification by employ- 

 ing higher powered eyepieces. 



In microscopic qualitative chemical analysis employ low-power 

 objectives which have been specially selected, if possible, because 

 of their long working distance and high penetrating power; 

 sacrifice resolving power for the convenience of being able to 

 thus obtain great depth of focus. 



