;6d DARK-GROUND ILLUMINATION WITH HIGH POWERS [Ce. II 



129. Centering and focusing the dark-field condenser. While 

 it is desirable to have the condenser well centered for bright-field 

 work it is absolutely essential in dark-field microscopy; and all dark- 

 field condensers worthy of the name have centering arrangements. 

 To center the condenser after it is put in the substage ring and pushed 

 up so that the upper end of the condenser is flush with the upper sur- 

 face of the stage, or very slightly above it, one uses a low objective 

 and ocular and focuses down until the top of the condenser is in 

 focus. There is a delicate ring on the upper face of the dark-field 

 condenser; this must be made to occupy the exact center of the field 

 by using the centering screws of the condenser mounting. This 

 centering answers for most work, but to center the homogeneous 

 immersion objective, or other high- power objective, and the condenser 

 where the field is so small that the ring on the condenser is consider- 

 ably larger than the field of the objective, one can make a minute 

 dot of white ink in the exact center of the upper face of the condenser 

 (fig. 48c, r). A crow-quill pen is good for this. Then some starch 

 or flour is scattered over the face of the condenser so that one can 

 focus upon it with the high power. For the centering the oil-immer- 

 sion objective it is not necessary to use any immersion liquid. The 

 condenser is then got as nearly centered as possible with the low 

 power, and then the high power is focused upon the condenser and it 

 is shifted if necessary until the central spot of ink is in the middle of 

 the field of the objective. The spot of white ink can be easily re- 

 moved by the use of a moist cloth or a moist piece of lens paper. 



Focusing the condenser on the specimen is accomplished thus: 

 A slide of the thickness required by the condenser is used. On it 

 is mounted some starch in water, and a cover-glass is added. A 

 drop of cedar oil is put on the top of the condenser and on the under- 

 side of the slide opposite the preparation. The slide is then put 

 in place and the condenser moved up if necessary until the top almost 

 touches the slide. If now one lights well and examines the specimen 

 with a low power there will be seen a spot or ring of light. By mov- 

 ing the condenser up and down one can find the position where the 

 spot of light on the specimen is the smallest and brightest, and that 

 will be the position of the best focus, and where the most favorable 



