Page:The History of the Standard Oil Company Vol 1.djvu/332

Rh by Dardonville, lampist, Broadway, New York. The gas used was that of the New Haven Gas Light Co., made from best Newcastle coal, and of fair average quality.

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.

From this table it will be seen that the rock-oil lamp was somewhat superior in illuminating power to Carcel's lamp of the same size, burning the most costly of all oils. It was also equal to the "Diamond Light" from a lamp of one-half greater power, and consequently is superior to it in the same ratio in lamps of equal power. The camphene lamp appears to be about one-fifth superior to it, but, on the other hand, the rock-oil surpasses the camphene by more than one-half in economy of consumption (i.e., it does not consume one-half so much fluid by measure), and it burns more constantly. Compared with the sylvic oil and the sperm, the rock-oil gave on the ground glass diaphragm the whitest disc of illumination, while in turn the camphene was whiter than the rock-oil light. By the use of screens of different coloured glass, all inequalities of colour were compensated in the use of the photometer, so that the intensity of light could be more accurately compared. Compared with gas, the rock