An incandescent bulb or mantle may be satisfactory both in brilliancy and color, but a further question is, How long will the filament or the mantle last, and at what point in deterioration should it be discarded? Tests during the first, the fiftieth, the hundredth, and other successive hours will tell us how much the intensity falls off. Just when a bulb or a mantle should be dismissed from service depends partly on the rate of deterioration, and partly on the prices of bulbs and current, of mantles and gas. Hardly less important than testing sources of light is the investigation of their reflectors and shades. As a rule our lamps are too brilliant, and in many cases they send their light in wasteful directions. It is a general and absurd practice to buy a dollar’s worth of light and then kill sixty cents’ worth of it with a thick opal or cut-glass shade. Examination with the photometer has revealed that many popular patterns of reflectors and shades are most ineffective, while those of the Holophane make, when kept scrupulously clean, send the light just where it does most good and at the lowest possible expenditure of energy. This theme has attention on page 78.[28] [28] A capital treatise on the subject of lighting, and the measurement of light, is Louis Bell’s “Art of Illumination.” New York, McGraw Publishing Co., 1902. $2.50. Its author (August, 1906) is preparing a new and revised edition. The Sky as a Field for Measurement. The sky has been the supreme field for measurements more refined from age to age. Professor William Stanley Jevons, in “Principles of Science,” says: “At Greenwich Observatory in the present day, the hundredth part of a second is not thought an inconsiderable portion of time. The ancient Chaldeans recorded an eclipse to the nearest hour, and even the early Alexandrian astronomers thought it superfluous to distinguish between the edge and centre of the sun. By the introduction of the astrolabe, Ptolemy and the later Alexandrian astronomers could determine the places of the heavenly bodies within about ten minutes of arc. But little progress then ensued for thirteen centuries, until Tycho Brahe made the first great step toward accuracy, not only by employing better instruments, but even more by ceasing to regard an instrument as correct. Tycho, in fact, determined the errors of his instruments, and corrected his observations. He also took notice of the effects of atmospheric refraction, and succeeded in attaining an accuracy often sixty times as great as that of Ptolemy.