contrast require a narrower cone, gaining thereby greater 

 depth of sharpness (penetration). The narrower the 

 cone the flatter the field appears. Very few objects 

 permit of a cone which fills the back lens of the objective 

 and in no case should the diameter of the iris diaphragm 

 appear to be larger than the diameter of the back lens 

 when the ocular is removed. 



When objectives of over i.o N. A. (see p. 25) (immer- 

 sion objectives) are used, the full aperture of the con- 

 denser cannot be utilized without immersing it, i. e., 

 placing a drop of oil between it and the lower surface 

 of the slide. This is seldom practiced in general labora- 

 tory work on account of its inconvenience, but it is 

 necessary to the most critical work. 



In working with artificial light it is always best to use 

 a bull's eye condenser. If no bull's eye is available, use 

 the concave mirror and so turn it that the image of the 

 light source appears in the centre of the back lens of the 

 objective when the ocular is removed. When the bull's 

 eye is used, select the plane mirror and so place the light 

 source and bull's eye that the image of the light source 

 appears natural size on a cardboard placed at the back 

 of the condenser. Remove the bull's eye and put the 

 light source in its place. Focus the substage condenser 

 so that the image of the light source appears in the plane 

 of the object. This is best seen by using a low power 

 objective and ocular. Now put the bull's eye in its 

 former position after removing the light to its original 

 place, or as before, so arrange the light source that a 

 sharp image of the light source appears on the back 

 of the condenser iris, or a card placed against it. This 

 will give an even illumination. A blue glass should also 

 be used beneath the condenser, unless the blue globe 

 mentioned on page 14 is used. This modifies theyello\v 

 artificial light. 



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