CH. If.] 



LIGHTING AND FOCUSING. 



49 



Fig. 52. r. Lamp with slit-opening in metal chimney, 

 stand. 3. Screen showing image of flame. 



Bull's eye on separate 



Whenever possible, the edge of the flame is turned toward the micro- 

 scope, the advantage of this arrangement is the greater brilliancy, due 

 to the greater thickness of the flame in this direction. 



§ 90. Mutual Arrangement of Lamp, Bull's Eye and Micro- 

 scope. — To fulfill the conditions given above, namely, that the object 

 be illuminated by the image of the source of illumination the lamp must 

 be in such a position that the condenser projects a sharp image of the 

 flame upon the object (Fig. 52), and only by trial can this position be 

 determined. In some cases it is found advantageous to discard the mir- 

 ror and allow the light from the bull's eye to pass directly into the conden- 

 ser. This method is especially excellent in photomicrography (see Ch. 

 VIII). 



§ 91. Illuminating the Entire Field.— With low objectives and 

 large objects, the entire object might not be illuminated if the above 

 method were strictly followed ; in this case, turn the lamp so that the 

 flame is oblique, or if that is not sufficient, continue to turn the lamp 

 until the full width of the flame is used. If necessary the condenser 

 may be lowered also. (See also § 80). 



REFRACTION AND COLOR IMAGES. 



\ 92. Refraction Images are those mostly seen in studying microscopic objects. 

 They are the appearances produced by the refraction of the light on entering and on 

 leaving an object. They therefore depend (a) on the form of the object, (b) on the 

 relative refractive powers of object and mounting medium. With such images the 

 diaphragm should not be too large (see \ 79). 



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