FIG. 4. INTENSITY CURVE, INCANDESCENT BODY GENERALIZED.
According to the measurements of Fi-aunhofer, Koenig, Brodhun, and others, the distribution of intensity in the spectrum of the sun and other incandescent bodies corresponds roughly to the curve in the accompanying diagram, Fig. 4. A comparison of the intensity-distribution in the spectra of the electric incandescent and arc lights with that of the sun gives the following results, calling unity the relative intensity of each with respect to the sun for sodium light (1) line) : The reflecting power of a polished plane sur face or the absorbing power of a thin film or plate of an absorbing medium can be determined for various colors by means of a spectro-photometer, by placing the mirror or the thin film in front of the adjustable slit T', and comparing the inten sity of the light after reflection or transmis.sion with the intensity of the light coming directly to the other collimator of the photometer. For ex
ample, a polished silver surface reflects 95 per cent. of the red light falling on it normally, while for the blue only 60 per cent. is reflected. A mirror of solid cyanine reflects 15 per cent. of the orange, 2 per cent., of the green, and 6 per cent. of the violet.
BIBLIOGRAPHY. The methods of observation and Bibliography. The methods of observation and of calibration, and the sources of error are fully treated in the following articles: Dimmer and Brodbun, Zeitschrift fur Instrumentenkunde, 1889, p. 42, and 1392, p. 137 (Berlin) ; Alurphy, Astro-Physical Journal, 1S97, p. 1 (Chicago) ; Brace, Philosophical Magazine, 1399, p. 420 (Lon don), and Astro-Physical Journal, 1900, p. 6 (Chicago) ; Capps, Astro-Physical Journal, 1900, p. 25 (Chicago) ; Tuckermann, Astro-Physical Journal, 1902, p. 145 (Chicago).