6 4 



This great range of intensity seriously compli- 

 cates the determination of the magnitude scale, 

 but photographic methods have been developed 

 with the 6o-mch reflector, and applied to a deter- 

 mination of standard magnitudes for all the stars 

 of Pickering's North Polar Sequence and numerous 

 other faint stars near the Pole. The scale thus 

 established, over a range of about 17^ magnitudes, 

 probably represents satisfactorily the degree of 

 homogeneity attainable by the use of uniform 

 methods employed with a single instrument. 



For the ready determination of stellar magni- 

 tudes over the whole sky, standard magnitudes 

 on an absolute scale, of a considerable number of 

 uniformly distributed stars, must be measured. 

 To this end observations of such standard stars 

 to the seventeenth magnitude in each of Kapteyn's 

 1 1 ^ Selected Areas on and north of the celestial 

 equator are now in progress. 



Magnitudes measured on ordinary photographic 

 plates give the intensity of the blue and violet light 

 of the stars. By using plates sensitive to red or 

 yellow, behind a yellow glass filter, the resulting 

 ' photovisual' magnitudes furnish a measure of 

 the intensity of the red or yellow light. As the 

 relative intensity of the blue and red light for any 

 object depends upon its color, a comparison of the 

 photographic and photovisual magnitudes gives at 

 once a measure of the color. The method is of 

 importance, as it can be applied to the faintest 

 stars that can be photographed. 



