THE MICROSCOPE 35 



clear cedar oil, free from dust and air-bubbles, on the 

 cover-glass, which must be perfectly dry. In this case, 

 by careful manipulation, the objective, after being 

 brought in touch with the oil by means of the coarse 

 adjustment, may be gradually lowered by the fine adjust- 

 ment until the object is focused; or lower the objective 

 until almost in touch with the cover-glass, and focus up. 

 High powers always require the use of a homogeneous 

 liquid between the cover-glass and the front lens of the 

 objective, to avoid loss of light by refraction. As a bundle 

 of rays disperses when entering a thinner medium from a 

 denser one, there is not sufficient light entering the objec- 

 tive to make objects discernible, when using high powers. 

 By the insertion of a liquid (inspissated oil of cedar) of 

 the same, or nearly the same, refractive index, a homo- 

 geneous connection is estabHshed between the cover-glass 

 and the objective, thus avoiding this loss and allowing a 

 bundle of light of sufficient power to enter the objective.' 



14. After using the oil-immersion lens, wipe the oil 

 off with lens-paper. If the oil sticks to the lens, wipe it 

 off with xylol, never with alcohol. At the same time 

 wipe the oil off the cover-slip. 



15. The microscope should stand on a firm table, 

 to avoid being shaken. The table should be low enough 

 so as not to necessitate bending the body. 



16. Always keep both eyes open. This saves the 

 eyesight. Beginners find this a difficuh rule to apply, 

 but with very httle practice and persistence it is easily 

 accomplished. Also use both eyes alternately. 



17. It is always well to move the object while bringing 



'For detailed description and diagrams see S. H. Gage The 

 Microscope. 



