CH. If] 



LIGHTING AND FOCUSING 



51 



one made of metal with a slit-opening covered with an oblong cover- 

 glass is more satisfactory, as the source of light is more restricted. 

 Very excellent results may be obtained, however, with the ordinary 

 bed-room lamp furnished with the usual glass chimney. 



The new acetylene light promises to be . excellent for micro- 

 scopic observation and for photo-micrograph}-. (See under photo- 

 micrography.) 



Fig. 53. 1. Lamp with slit-opening in metal chimney. 2. Bull's eye on 

 separate stand. 3. Screen showing image of flame. 



Whenever possible the edge of the flame is turned toward the 

 microscope, the advantage of this arrangement is the great brilliancy, 

 due to the greater thickness of the flame in this direction. 



§ 94. 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. 53), and only by trial can this posi- 

 tion 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 photomicro- 

 graphy (see Ch. VIII). 



§ 95. 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 



