CH. //.] 



LIGHTING AND FOCUSING. 



49 



Fig. 52. 1. Lamp with slit-opening in metal chimney. 2. Bull's eye 011 separate 

 stand. 3. Screen showing image of flame. 



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 

 mirror and allow the light from the bull's eye to pass directly into the 

 condenser. 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 

 ma}' be lowered, and the concave mirror used. ( See also ^ So). 

 REFRACTION AND COLOR IMAGES. 

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

 The}' 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 2 79). 



If the color and refractive index of the object were exactly like the mounting 

 medium it could not be seen. In most cases both refractive index and color differ 

 somewhat, there is then a combination of color and refraction images which is a 

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