ASTRONOMY: F. H. SHARES 
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 photo- 
graphic and photo visual magnitude for stars near the North Pole. Al- 
though 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 meth- 
ods, there is serious difficulty in evaluating the functional relation con- 
necting 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 rela- 
tion ; 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 com- 
parison of quantities which, relatively, are very large and very small. 
In the present case, the range of brightness actually covered in the de- 
termination of the photographic scale is about 17|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 the standardization of the 
photographic effect in terms of intensity have as their principle the 
production of two series of images with a known variation of the in- 
tensity between the exposures. This in itself is not easily accom- 
plished, at least with assurance of freedom from systematic error. A 
variation of the total energy by a change in the exposure time imme- 
diately introduces the photographic difficulties that we wish to avoid; 
a change in the intensity by a reduction of the aperture changes the 
diffraction pattern of the optical image, and it is not a priori certain 
