442 



SCIENCE 



[N. S. Vol. XLII. No. 1083 



spectrum of every star brighter than the 

 ninth magnitude. It would be very desir- 

 able to determine the proper motions of all 

 these stars. If all the material available is 

 examined it should be possible to do this to 

 a very large extent. 



PHOTOMETRY AND COLOE 



For the more distant parts of the heav- 

 ens proper motions are an uncertain guide, 

 and we must depend on what can be learned 

 from the light of the stars by means of 

 stellar photometry, determinations of color, 

 and studies of stellar spectra. Speaking 

 generally, we attempt to discover from the 

 nearer stars sufficient about their intrinsic 

 luminosities to enable us to use the appar- 

 ent magnitude as an index of the distances 

 of the stars which are further away. The 

 most striking example is found in Professor 

 Hertzsp rung's determination of the dis- 

 tance of the small Magellanic cloud. From 

 a knowledge of the characteristics of the 

 Cepheid variables found in this cloud by 

 Miss Leavitt, and their apparent magnitude, 

 he deduced the distance of the cloud as 

 10,000 parsecs. 



Much attention has been given of late 

 years to stellar photometry. In 1899 Pro- 

 fessor Pickering published the Revised 

 Harvard Photometry giving the magni- 

 tudes of all stars brighter than 6.5 m. In 



1907 Messrs. ]\Iiiller and Kempf completed 

 a determination of 14,199 stars of the north- 

 ern hemisphere brighter than 7.5 m. In 



1908 a catalogue of 36,682 stars fainter 

 than 6.5 m. was published at Harvard. 

 These determinations derive additional im- 

 portance as they give the means of stand- 

 ardizing estimates of magnitude made by 

 eye, particularly the many thousands of the 

 Bonn Durchmusterung. 



By the labors of Professor Pickering and 

 his colleagues at Harvard, Professor 

 Sehwarzschild, Professor Parkhurst at 



Yerkes, Professor Scares at Mount Wilson, 

 and others, the determinations of the mag- 

 nitudes of stars by photography has made 

 rapid strides. As yet no complete cata- 

 logues of photographic magnitude corre- 

 sponding to the Revised Harvard Photom- 

 etry have been published, though consider- 

 able parts of the sky and special areas, such 

 as the Pleiades, have been carefully studied. 

 The determination of the photographic mag- 

 nitudes of any stars which may be required 

 is, however, a comparatively simple matter 

 when the magnitudes of sufficient standard 

 stars have been found. A trustworthy and 

 uniform scale has been to a large extent 

 secured by the use of extra-focal images, 

 gratings and screens in front of the object 

 glass, and the study of the effects of differ- 

 ent apertures and different times of ex- 

 posure. 



At Harvard and Mount Wilson, standard 

 magnitudes of stars near the north pole 

 have been published extending to nearly 

 the twentieth magnitude. In the part of 

 the range extending from 10.0 m. to 16.0 m. 

 these agree very satisfactorily. There is, 

 however, a difference of 0.4 m. in the scale 

 between 6.0 m. and 10.0 m. which needs to 

 be cleared up. 



A uniform and accurate scale of magni- 

 tude is of fundamental importance in counts 

 of the numbers of stars. Such counts aim 

 at the determination of two things: (1) 

 how the numbers vary in different parts of 

 the sky, and (2) what is the ratio of the 

 number of stars of each magnitude to that 

 of the preceding magnitude in the same area 

 of the sky. The counts of stars from the 

 gauges of Sir William and Sir John Her- 

 scliel, those of the stars contained in the 

 Bonn Durchmusterung, those made by 

 Professor Celoria, and the recent counts of 

 the Franklin-Adams plates by Dr. Chap- 

 man and Mr. Melotte, all agree in showing 

 a continuous increase of stars as we proceed 



