CH. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS y6g 



(2) The dark-field condenser must be put into the ring of the sub- 

 stage, and raised until its top is flush with the upper surface of the 

 stage. The condenser is then accurately centered. If there is an 

 iris diaphragm below the condenser it should be made wide open. 



(3) A homogeneous immersion objective, containing a suitable 

 reducing diaphragm (i.e., one giving 0.80 to 0.90 N.A.), is screwed 

 into the opening in the nose-piece of the microscope. 



(4) Slides and cover-glasses of the proper thickness are cleaned 

 thoroughly and put in position for rapid handling. 



(5) The preparation to be examined blood, saliva, etc. is put 

 on the slide and covered. The cover-glass is sealed with mineral or 

 with castor oil, or with shellac cement and the slide is labeled. 



(6) A drop of homogeneous oil is put upon the upper face of the 

 condenser and one or more drops on the under side of the slide op- 

 posite the preparation. One or more drops are then put on the 

 cover-glass. Plenty of oil is used so that when the slide is moved 

 around in the examination good immersion contact will be maintained. 



(7) The slide is placed on the stage so that the oil makes immer- 

 sion contact with the underside of the slide and the top of the con- 

 denser. The condenser may need to be raised slightly to make the 

 contact perfect, or it may need to be lowered slightly so that the slide 

 will rest on the stage. 



(8) The microscope and lamp are arranged to give the best light 

 by having the parallel beam fall on the mirror, then the mirror is 

 turned slightly until there appears a brilliant point of light in the oil 

 on the cover-glass. The objective is then lowered until it dips into 

 the oil. 



(7) The microscope is then focused and the light made as brilliant 

 as desired by slightly moving the mirror. In focusing it will be seen 

 that when the objective is near the object the whole field will look 

 bright, but as the focus is reached the specimen alone will be bright 

 and the field dark. 



(10) Dark-field microscopy requires more skill and accuracy of 

 manipulation than does bright-field microscopy, but the increased 

 visibility pays for all the trouble required. It is best to work in the 

 evening or in a dark or dimly lighted room for then the eyes are ad- 



