The distance between the standard candle, and the illuminator sought to be determined, was constantly 150 inches—the photometer traversed the graduated bar in such a manner as to read, at any point where equality of illumination was produced, the ratio between the two lights. I quote only single examples of the average results, and with as little detail as possible, but I should state that the operation of the photometer was so satisfactory that we obtained constantly the same figures when operating in the same way, evening after evening, and the sensitiveness of the instrument was such that a difference of one-half inch in its position was immediately detected in the comparative illumination of the two equal discs of light in the dark chamber. This is, I believe, a degree of accuracy not before obtained by a photometer. TABLE OF ILLUMINATING POWER OF VARIOUS ARTIFICIAL LIGHTS COMPARED WITH JUDD’S PATENT CANDLES AS A UNIT SOURCE OF LIGHT RATIO TO CANDLE—1 Gas burning in Scotch fish-tail tips, 4 feet to the hour 1 : 5.4 Gas burning in Scotch fish-tail tips, 6 feet to the hour 1 : 7.55 Gas burning in Cornelius fish-tail tips, 6 feet to the hour 1 : 6.3 Gas burning in English Argand burner, 10 feet to the hour 1 : 16 Rock-oil, burning in 1–inch wick camphene lamp, consuming 1¾ ounces of fluid to the hour 1 : 8.1 Carcel’s mechanical lamp, burning best sperm oil, 2 ounces of fluid to the hour, wick ⅞ of an inch 1 : 7.5 Carcel’s mechanical lamp, burning best sperm oil, 2 ounces of colza oil to the hour, wick of ⅞ an inch 1 : 7.5 Camphene lamp (same size as rock-oil above) burning best camphene, 4 fluid ounces per hour 1 : 11 “Diamond Light” by “sylvic oil,” in 1½–inch wick, 4 ounces per hour 1 : 8.1