The conventional method of making pecuniary comparisons between different sources of artificial light consists in simply calculating the cost of developing a certain number of candle-hours of light--_i.e._, a certain amount of standard candle-power for a given number of hours--on 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- lamps. The standard of illumination adopted for the table is one which is only gaining general recognition where incandescent gas or acetylene lighting is available, though in exceptional cases it has doubtless been attained by means of oil-lamps or flat-flame gas-burners, but very rarely if ever by means of carbon-filament electric glow-lamps, or candles. It assumes that the occupants of a room do not wish to be troubled to bring work or book "to the light," but wish to be able to work or read wheresoever in the room they will, without consideration of the