the amount and direction of light, the displacement of 

 condenser when it is not used, etc. 



The most simple form, largely used for instruments 

 for laboratory and everyday work, is one which has 

 attached to its lower side an iris diaphragm for regulat- 

 ing the amount and angle of light and to which is 

 attached a swinging arm to receive blue glass for use 

 with artificial light, or stops for dark ground or oblique 

 illumination. A vertical screw motion gives a service- 

 able means of adjustment and when at its low r est limit 

 of adjustment it may be swung out of the optical axis. 



The most complete 

 form is that shown in 

 Fig. 5 1 a, b and c, .whfch 

 has adjustments for obtain- 

 ing every modification and 

 character of illumination, 

 with rack and pinion for 

 vertical adjustment and 

 swinging the condenser 

 and iris diaphragm out of 

 the way if it is not desired 

 to use them. 



The condenser should not be used on very low 

 power .objectives as it is distinctly harmful and the 

 mirror alone provides ample illumination. 



The following description of the parts of the com- 

 plete substage will facilitate its use. 



Fig. 5 la. Complete Substage. 

 Front view. 



156 



