SPECTBO-PHOTOMETRY. 424 SPECTRO-PHOTOMETRY. SPECTRO-PHOTOMETRY (from Lat. spec- trum, appearance, image, apparition, + Gk. ipwt, pMs, light + -/xerpia, -metria, measurement, from /iirpoy, metron, measure), or Spectral Photometby ( see also Spectroscopy and Photom- etry). The study of the relative intensities of light of various colors from the same source or from different sources. Not only, as an ordinary photometry, may the relative intensities of two sources of white light or of monochromatic light be compared, but spectrophotometers are pro- vided with dispersing prisms so arranged that the various colors of one beam of white light may be compared with the corresponding colors of an- other. The general method is to bring some of the light from one source and some from the other Fig. 1. DOUBLE PRIBM, BBACE'B 8PECTRO-PHOTOMETEB. source side by side in the same field of view, and by suitable means to alter the intensity of either beam (or of both) in a known degree until a "match' or photometric equality is secured; this condition of equality being determined by the vanishing of the line of separation between the two portions of the field of view, illuminated re- spectively by the two sources to be compared. There is also a so-called 'method of contrast' de- vised by Lummer and Brodhun. The most recent and efficient form of spectro- photometer is that devised by Professor Brace of metrically situated with respect to the prism and provided with adjustable slits through which the light from the two sources enters the optical sys- tem of the photometer. The amount of light entering the sj'stem through either collimator de- pends upon the width of its slits. The width of one slit, say T, is, after the initial adjustments of the instrument, kept fixed througliout an.v one series of observations, while the width of the other collimator-slit (T') may be altered at will to secure a match in intensity of the two beams. Light of the same wave-length (i.e. of the same color) is thus brought by direct transmission from the collimator T, and after reflection at the silver strip from T', to the same focus in the observing telescope R. When the eyepiece is removed and the prism _ viewed through a slit in the focal plane of R, the eye sees three fields (as in Fig. 3 ) , the central one, ABCD illuminated by light from the right collimator, T', and the upper and lower ones, ACF and BDG, by light from the left colli- mator, T. The fields meet in the sharp edges of the silver strip. A match in intensity is secured by altering the width of the slit T', which is controlled by an accurately turned screw carrying a graduated drum so that the width of T' can be altered at will by a known amount and thus the total amount of light passing through this collimator can be changed as desired. Tlie intensities of two beams will be inversely pro- portional to the slit-widths required for a match between them and the light coming through the fixed slit T. (This simple relation is not quite true; the deviations from it are treated in the articles cited below.) The amount of light coming through the collimator with the fixed slit may be altered without changing the width of the slit by means of a rotating disk mounted direct- ly in front of the fixed slit, so as to cut off the light during a certain fraction of each revolution. This device greatly increases the range of the in- strument. By turning the telescope R through a small angle the various colors of the spectrum may be brought into the field of view in succession, and the relative intensities of the two sources for each color determined. FlO. 3. FIELD TIEW OF BPECTRO-PHOTOMETEB. Fig. 2. OPTICAL BTSTEM, BRACE'S SPECTRO-PHOTOMETEE. the University of Nebraska. It consists essential- ly (see Fig. 1) of a double prism P with a nar- row silvered strip SS on the face AD of the right half. T and T' are two similar collimators sym- Wa-^e -length Fig. 4. INTENBITT CURVE, INCANDESCENT BODY GENERALIZED. According to the measurements of Fraunhofer, Koenig, Brodhim, and others, the distribution of intensity in the spectrum of the .sun and other incandescent bodies corresponds roughly to the
