CH. II~\ 



LIGHTING AND FOCUSING 



61 



vantageous to discard the mirror and allow the light from the bull's 

 eye to pass directly into the condenser. In most cases no bull's eye 

 need be used. The proper distance of the lamp from the mirror and 

 the proper elevation of the condenser give the required results. 

 The position of lamp and condenser can be determined by trial in 

 each case. 



§ 1 06. Illuminating the Entire Field. — With low objectives 

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



Fie 60. 1. Lamp with sHt-opening in metal chimney. 2. Bull's eye on 

 separate stand. 3. Screen showing image of flame. 



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, and the concave mirror used. (See 

 also § 95.) 



REFRACTION AND COLOR IMAGES 



\ 107. 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 \ 94). 



If the color and refractive index of the object were exactly like the mount- 

 ing 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 



