CH. X] DRAWING WITH HOUSE CURRENT 347 



and the microscope at right angles. Use a level (fig. 160) and make 

 sure that the condenser tube is horizontal, and the axis of the con- 

 denser at the same level as the center of the mirror. Place a disc 

 of blackened asbestos or tin of about 12.5 cm. (5 in.) in diameter 

 just behind the condenser as shown in fig. 193-198. This is easily 

 done by making a hole of the proper size in the disc to go over the 

 condenser tube (fig. 195). If now the current is turned on and the 

 arc established the light will extend from the condenser to the plane 

 mirror and be reflected by it, if it is set at 45 degrees, up into the 

 substage condenser. From the lower face of the substage con- 

 denser a part of the light is reflected back to the mirror and from 

 the mirror back toward the lamp, and is received by the black disc 

 over the condenser tube. The mirror should then be turned until 

 the spot of light enters the lamp condenser. The mirror will then 

 be in position to reflect the light along the optic axis of the micro- 

 scope. 



If the microscope is in focus on the object, the light will traverse 

 the objective and ocular and be reflected down upon the drawing 

 surface by the mirror or prism beyond the ocular. 



By changing the mirror slightly while watching the circle of light 

 on the drawing surface, the best illumination can be obtained. 



498. Getting the light through the microscope with the con- 

 cave mirror. One proceeds exactly as described above, only the 

 light reflected back to the black disc on the lamp condenser tube 

 will be a crescent instead of a circle. The middle part of the cres- 

 cent can be reflected into the lamp condenser and then the light will 

 pass through the microscope and be reflected down upon the draw- 

 ing paper, provided the microscope and the arc lamp are at right 

 angles and at the proper level. 



499. Substage condenser. Use the substage condenser with 

 objectives of 16, 12, 10, 8, 6, 4, and 2 mm. focus. For objectives 

 lower than the 16 mm. the substage condenser is turned aside. 



With different objectives and slides of different thickness the 

 substage condenser is changed somewhat in position to get the best 

 light on the object and to light the entire field. 



