16 ELEMENTARY CHEMICAL MICROSCOPY 



objective is clean yet the field shows specks of dirt and appears 

 blurred, the dust and dirt will be found to be on the disk carry- 

 ing the cross-hairs or micrometer scale. Exceeding great care 

 is required in cleaning cross-hairs and micrometer plates resting 

 upon the diaphragm of the ocular and should be undertaken 

 only by a person having patience, care and steady nerves. 



Use low oculars first and confine the work whenever possible 

 to medium powers. Have recourse to high-power oculars only 

 as a last resort, since they cut down the light to such an extent 

 as to cause fatigue and eye-strain. 



Always look into a microscope with both eyes open. 



In the study of flat preparations between slides and cover 

 glasses, the general rule is to obtain the proper magnification 

 chiefly by means of the objective, using a low-power ocular. But 

 in the case of irregular surfaces or curved and heaped-up drops 

 of liquid, the reverse is essential and low-power objectives (having 

 long free working distance) and high oculars must be adopted. 

 The latter procedure is also indicated when employing dark- 

 ground illuminators or ultra-condensers, namely, increase the 

 magnification by the ocular. 



Limit of Magnification. — A consultation of the tables of 

 magnification given in the catalogues of the leading makers of 

 microscopes and microscope lenses will show that with the mod- 

 ern compound microscope employed in the usual manner with 

 stock achromatic objectives and Huygenian oculars, a magni- 

 fication as high as 1500 to 2000 may be obtained, and that with 

 stock apochromatics and compensating eyepieces this may still 

 further be increased to 3000, the upper limit of listed combi- 

 nations. 



Theoretically there is no limit to the magnification which 

 may be obtained. But this must not be confused with resolving 

 power which enables us to see things clearly and permits differ- 

 entiating one part or structure from another. Great magnifica- 

 tion avails us nothing if the image be blurred and irrecognizable. 

 A little thought will show that there must be a limit to the 

 resolving power practically available beyond which we can- 

 not go. 



