The light produced by wax and tallow candles, and by oil and gas under combustion, is yellow in color ; this is the reason why we can scarcely distinguish between blue and green colors at night by artificial illumination. A correspondent of the London Mechanics' Magazine makes an inquiry regarding the possibility of obtaining artificial white light, by making it pass through a series of glasses tinted according to the prismatic spectrum, neglecting the yellow ray, of course. A very great improvement in the color of the artificial light could easily be effected by employing globes or shades of a very faint purple color. Purple is composed of the red and blue rays of the spectrum, which, properly combined with the yellow ray, produce white.