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CH. X] DRAWING WITH HOUSE CURRENT 347
and the microscope at right angles. Use a level (fig. 1 60) and make sure that the condenser tube is horizontal, and the axis of the condenser 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 condenser 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 microscope.
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 concave 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 crescent can be reflected into the lamp condenser and then the light will pass through the microscope and be reflected down upon the drawing 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.