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ASTRONOMY: F. H. SEARES 309 ABSOLUTE SCALES OF PHOTOGRAPHIC AND PHOTOVISUAL MAGNITUDE By Frederick H. Scares MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON Presented to the Academy. April 14. 1915 The first extensive photometric investigation undertaken with the 60-inch reflector has been the determination of absolute scales of photographic and photovisual magnitude for stars near the North Pole. Although the instrument is best adapted for the observation of objects fainter than the tenth magnitude, it can also be used for the brighter stars; and it thus becomes possible to secure, on a uniform system of color, results covering the entire range of stellar brightness at present known. In any photometric problem involving the use of photographic methods, there is serious difficulty in evaluating the functional relation connecting the intensity of the light acting on the plate with the observed photographic effect. The photographic process is too complicated and too sensitive to slight fluctuations in the conditions which determine its action to permit of the application of anything like a general relation; and it is necessary to standardize or calibrate the images appearing on each plate which is to contribute toward an absolute scale. Besides photographic difficulties, there is another which is serious, namely, that encountered in all physical observations involving the comparison of quantities which, relatively, are very large and very small. In the present case, the range of brightness actually covered in the determination of the photographic scale is about 171 mags.; the intensity ratio for the brightest and the faintest of the stars observed is therefore of the order of 1 to 10,000,000, and consequently the opportunity for an accumulation of error in passing from the one extreme of brightness to the other is great. The methods commonly employed for