PRACTICAL DETAILS 127 



C. Arrangement of the preparation to be examined. 



1. The preparation to be examined should be mounted on a slide and 

 covered with a cover-glass. 



(a) The slide should be of crystal glass free from flaws and absolutely 

 clean, because any dust or dirt will seriously interfere with the ob- 

 servation. 



Slides and cover-glasses should be washed in acid rinsed in distilled water 

 and kept in alcohol (p. 130). When required for use, it is advisable in order 

 to ensure cleanliness to paint the slide with a layer of collodion, which can 

 be peeled off just before it dries. 



Dust which falls on the cover-glass during the examination interferes 

 with the satisfactory lighting of the preparation, and if the observation 

 be prolonged the cover-glass should be washed or dusted from time to 

 time. 



(b) To secure the most satisfactory illumination the slide should be of a 

 thickness suitable to the particular apparatus in use (all dark-ground con- 

 densers are marked with a number indicating, the thickness of slide to be 

 used generally about 1*4 mm.). When working with sunlight it is absolutely 

 necessary that slides of the exact thickness indicated on the condenser should 

 be used ; but with the sources of light ordinarily employed this precision is 

 of less importance, and one-third of a millimetre one way or the other is a 

 matter of no great moment. 



The thickness of the cover-glasses should correspond with the correction 

 of the objective (p. 119). 



2. There should be continuity between the media through which the light 

 passes, so that refraction may take place under the best conditions ; a large 

 drop of very fluid immersion oil should therefore be placed between the 

 condenser and the slide. 



An inferior quality of oil is a frequent cause of failure. The oil should be quite 

 fluid, absolutely homogeneous, contain no air bubbles, and be used in sufficient 

 quantity to completely fill the space between the lens and the condenser. 



3. The film to be examined should be as thin as possible, uniform and free 

 from air bubbles. If the material be sufficiently fluid and viscous to keep the 

 slide and cover-glass together the preparation may be examined without any 

 addition. In the contrary case, dilute the material in a drop of blood serum, 

 aqueous humour or ascitic fluid ; water or normal saline solution may be 

 used but these solutions have the disadvantage that they alter the shape 

 and interfere with the vitality of the cells. 



If the experiment is to be prolonged it is advisable to lute the edge of the 

 cover-glass with a little vaseline or paraffin to prevent evaporation. 



D. Focussing the microscope. 



For dark-ground illumination work a dry lens (No. 7, 8, or 9) may be used 

 (though an immersion lens is better) and a high eyepiece (No. IV. or Zeiss' 

 compensating ocular 18). 



To obtain a quite black background, special objectives can be employed in the 

 mounting of which a carefully centered diaphragm is suspended to intercept marginal 

 rays : these objectives (Leitz, Zeiss) give remarkably distinct images. 



A certain amount of skill which can only be obtained with practice is 

 required to get satisfactory results. 



1. With a dry objective. The lighting being satisfactory, the apparatus 

 centered and the preparation fixed with the clips, the eye is applied to the 

 tube of the microscope which is then slowly lowered. At first there is a 



