useful life of an electrode can be easily determined by burning it for a stated length of time, noting the length consumed and comparing it with the length available for burning.

CHAPTER III.
PROJECTION.

Setting and Adjustment of Carbons.

Setting and Adjustment of Carbons.—To project a picture upon a screen properly is an art and requires close study and some knowledge of all the factors involved. The most important factor is that of the light. Electric light is so universally used at the present time that it is hardly necessary to mention the other sources of illumination.