An Improved Method for Measuring Extinction.

A practical method is described for obtaining accurate extinction coefficients and xero-point terms for both magnitudes and colors. All necessary observations may be secured in a short time, of the order of 30 minutes, and the resulting extinction coefficients have an accuracy of about 2 per cent for magnitude and 3 per cent for colors. The use of nightly values of the coefficients, determined in this manner, results in higher-quality photometry and permits the use of less transparent skies than is the case when mean values are used. The relationships between the color coefficients and the magnitude coefficient are noted for a wide range of observing conditions.