MICROSCOPE ACCESSORIES 



223 



The electric illuminating devices include arc and 

 filament lamps. The electric arc furnishes the most 

 intense illumination, and 

 is especially useful in micro- 

 photography, owing to the 

 actinic quality of this light. 

 Arc lamps can only be used 

 in conjunction with a rheo- 

 stat or current control de- 

 vice, and one must look to 

 adjustment of the carbons 

 at frequent intervals. 

 While the light from fila- 

 ment lamps is not' as intense 

 as that of the electric arc, 

 it is sufficient for practically 

 all purposes. Low-voltage 

 types of filament lamps 

 require a rheostat or resist- 

 ance device. High-voltage 

 lamps are more convenient 

 in that they can be directly 

 connected with the source 

 of current. Condensing 

 lenses are usually inter- 

 posed between the lamp 

 and the microscope, so that 

 the light rays may be con- 

 centrated within a small 

 area. In long-continued 

 observations, electric illumination is often more desir- 

 able than daylight because of its constant intensity 

 and the ease with which it may be regulated. 



