SOUECES OF ILLUMINATION 



95 



of the source of light may be varied at will. This has in 

 practice been found to be a considerable advantage, as the 

 diaphragm is very close indeed to the source of light, and it is 

 therefore possible to vary exactly the degree of illumination 

 without altering the adjustments of the microscope. The 

 necessary resistances are mounted at the back of the lamp, the 

 resistance- wire being wound 

 on vertical tubes. To facili- 

 tate the lighting of the lamp, 

 a portion of the resistance 

 may be cut out by the spring 

 lever (shown on the left-hand 

 side of the figure). In this 

 type the resistances are not 

 carried in the lamp body, but 

 are separated from it. The 

 unavoidable heating that 

 occurs in the resistances does 

 not therefore tend to raise the 

 temperature of the other parts, 

 so that adjustment of the 

 illuminating portion may be 

 more easily effected. The 

 whole appliance is well made 

 and finished, and has all 

 the necessary arrangements 

 for varying height and 

 inclination. It is supplied 

 with a set of the necessary 

 colour-screens. 



The Electric Arc.Oi 

 the higher power illuminants 

 available the electric arc is, 



both theoretically and practically, almost an ideal one. From 

 the point of view of practicability it is at the present day 

 probably the most convenient, as nearly every laboratory and 

 many houses have a supply of electricity ; and from the 

 optical point of view it has the advantage that the source of 

 light more nearly approaches the much- desired optical point 

 than any other. 



FIG. 28. Mercury-vapour Lamp. 



