In view of the facts as to dissemination and diffusion, or
the difference between sheer illuminating power and useful illuminating
effect, which have just been elaborated, and in view of the different
intensities of the different unit sources of light (which range from the
single candle to a powerful large incandescent gas-burner or a metallic
filament electric lamp), such a method of calculation is wholly illusory.
The plan adopted in the following table may also appear unnecessarily
complicated; but it is not so to the reader if he remembers that the
apparently various amount of illumination is corrected by the different
numbers of illuminating units until the amount of simple candle-power
developed, whatever illuminant be employed, suffices to light a room
having an area of about 300 square feet (_i.e._, a room, 17-1/2 feet
square, or one 20 feet long by 15 feet wide), so that ordinary print may
be read comfortably in any part of the room, and the titles of books,
engravings, &c., in any position on the walls up to a height of 8 feet
from the ground may be distinguished with ease. The difference in cost,
&c., of a greater or less degree of illumination, or of lighting a larger
or smaller room by acetylene or any other of the illuminants named, will
be almost directly proportional to the cost given for the stated
conditions.
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