BACTERIOLOGICAL TECHNIC 49 



can be seen by looking at the instrument from one side 

 and watching the reflection of the objective in the cover 

 glass. Then, with the eye at the ocular, slowly focus 

 up. Do not focus down with the eye at the ocular, as 

 the lens may then come into violent contact with the 

 object, destroying the latter and injuring the lens. 



11. For living or transparent objects use as little 

 light as possible. For stained or opaque objects more 

 light is necessary. 



12. Do not use higher powers than are necessary. 



13. To use the oil -immersion lens, place a drop of 

 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 

 adjustment until the object is focused; or better, 

 lower the objective until almost in touch with the cover 

 glass, and focus up. High powers 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 enter- 

 ing a thinner medium from a denser one, there is not 

 sufficient light entering the objective to make objects 

 discernible, when using high powers without oil. By 

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

 nearly the same refractive index as glass, a homogene- 

 ous connection is established between the cover glass 

 and the objective, thus avoiding loss of light and allow- 

 ing a bundle of light of sufficient power to enter the 

 objective.' 



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

 Microscope. 



