62 EXPERIMENTS WITH A SUBSTAGE CONDENSER [Ch. II 



The greatest brilliancy comes by having the image of the lamp 

 flame on the object and by having as great a thickness as possible 

 of the flame toward the microscope. 



With low powers the face of the flame must be toward the micro- 

 scope in order to light the entire field, but with moderate powers it 

 may be oblique, and with the highest powers the edge of the flame 

 should face the microscope, as this utilizes the greatest thickness of 

 the flame. Make sure that the flame is centered by moving the mirror 

 or the lamp (fig. 44). 



As the light is diverging from the lamp flame it is usually better 

 to employ the concave mirror in order to overcome, in part, this diver- 

 gence, and light a larger field. In some cases it may be necessary 

 also to lower the condenser somewhat to get the entire field lighted. 

 A gas lamp with an incandescent mantle gives good results, also an 

 acetylene flame. Proceed in general as with the kerosene lamp. 



§ 109. Electric lamp. — If an electric bulb is used it should be 

 frosted for low powers; or what is better, a clear bulb should be used 

 and a piece of ground-glass should be put over the hole in the screen 

 between the lamp and the microscope (fig. 58). The lamp should be 

 close to the ground-glass. 



For moderate and high powers the ground-glass is excellent also. 

 For lighting the entire field it may be necessary to use the concave 

 mirror and also to lower the condenser. 



For the highest powers a single filament of the incandescent lamp 

 is focused on the object and carefully centered. For lighting the 

 entire field lower the condenser slightly and thus spread the light 



(fig- 42). 



Both mirrors should be tried alternately, also raising and lowering 

 the condenser a small amount. One can thus find which arrangement 

 gives the best possible results in difficult cases. While it may take 

 some time to try the different things, in the long run it will pay in the 

 satisfaction given by the more perfect image. 



For the electric arc with the microscope see under drawing and 

 photography (Chs. VI-VII). 



§ 110. Aperture and diaphragm. — It is to be remarked that with 

 a very small source of light the entire aperture of the objective may be 



