the assumption that as many separate sources of light are employed as may
be required to bring the combined illuminating power up to the total
amount wanted. 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. Nevertheless, it should be recollected that when the
conventional system is retained--useful illuminating effect being
sacrificed to absolute illuminating power--acetylene is made to appear
cheaper in comparison with all weaker unit sources of light, and dearer
in comparison with all stronger unit sources of light than the
accompanying table indicates it to be. In using the comparative figures
given in the table, it should be borne in mind that they refer to more
general and more brilliant illumination of a room than is commonly in
vogue where the lighting is by means of electric light, candles, or oil-