MOUNT WILSON SOLAR OBSERVATORY. 215 



fainter objects. The use of several different combinations of screens 

 and diaphragms afforded a number of independent determinations of 

 the scale, and gave results which show that the reduction constants 

 are correct within a few hundredths of a magnitude. The separate 

 scales are in good agreement, but show in each instance a considerable 

 divergence from the Harvard results. Combining the results for the 

 bright stars with those previously found for the faint stars, the differ- 

 ence between Mount Wilson and Harvard is approximately as follows : 



Mt. W. - H. = +0'?37-0'?070 (ILO-M) 



Both scales are referred to the international zero-point. The com- 

 parison covers magnitudes 2.0 to 15.5, but for M > 10 the last term of 

 the formula is to be disregarded. 



PHOTOGRAPHIC MAGNITUDES FOR THE SELECTED AREAS. 



The photogi-aphs of the Selected Areas form a part of a program 

 which has for its purpose the determination of standards of magni- 

 tude for faint stars in each of the areas on and north of the celestial 

 equator. In order that comparisons may be made with other inves- 

 tigations, the brighter stars will be 

 included, and to this end it is pro- 

 posed to establish the photographic 

 scale for each area over an interval 

 whose magnitude limits will be ap- 

 proximately 8.5 and 17.0. The in- 

 vestigation will be limited to stars within 10' of the central star of 

 each area. The relative magnitudes are to be determined by means 

 of exposures with diaphragms and a wire-gauze screen. The arrange- 

 ment of plates and exposures for each area is shown herewith. The 

 results will be reduced to the international zero-point by inter-com- 

 parison and by comparisons with the Pole. Of the 460 photographs 

 required for the relative magnitudes, 199 have been obtained. Each 

 plate will be measured twice. The measurement of 95 plates has been 

 completed and 22 others ha,ve been measured once. The reductions 

 are well advanced. 



BRIGHTNESS OF THE NIGHT SKY. 



Mr. van Rhyn has made a series of measures of the brightness 

 of the sky at night with the photometer described in Publication 

 No. 22 of the Astronomical Laboratory of Groningen. The results, 

 in harmony with those of Abbot and Yntema, indicate that the 

 brightness of the background of the sky is not due exclusively to 

 direct starlight. Assuming the value of the starlight for the higher 

 galactic latitudes given by the Groningen observations, Mr. van Rhyn 

 has sought to measure the brightness of star light in the Milky Way 



