364 TRANSACTIONS OF SECTION A. 



f)reparatioii at Harvard College, by Miss Cannon under Prof. Pickering's direc- 

 tion, a catalogue giving the type of spectrum of every star brighter than the 

 ninth magnitude. It would be very desirable to determine the proper motions 

 of all these stars. If all the material available is examined it should be pes 

 sible to do this to a very large extent. 



Photometry and Colour. 



For the more distant parts of the heavens 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 colour, 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 apparent 

 magnitude as an index of the distances of the stars which are further away. 

 The most striking example is found in Prof. Hertzsprung's determination of the 

 distance of the small Magellanic Cloud. "Visitors to Australia last year may 

 have seen in the sky two faint patches of light which look like pieces torn off 

 the Milky Way. These are called the Magellanic Clouds. In the small cloud 

 Miss Leavitt found 25 variable stars of special character known as Cepheids. 

 They are all very faint stars between 11'2 m. and 16"4 m. on the photographic 

 scale. The periods of their light variation range from 1'25. days to 127 

 days. Miss Leavitt found that a linear relationship existed between the 

 logarithm of the period and the apparent magnitude. As these stars all belong 

 to the Magellanic Cloud they are at the same distance, and thus there is a 

 relationship between the period of light variation and the intrinsic magnitude. 

 Prof. Hertzsprung found in Boss's catalogue 13 variable stars of similar class 

 of known proper motion. He deduced their mean distance by using the solar 

 motion, and from this calculated the mean intrinsic luminosity. He thus found 

 that Cepheid variables with a period of 6'6 days are 600 times as luminous as 

 the Sun, and have an absolute magnitude of — 7'3 m. But from Miss Leavitt's 

 observations similar stars in the small Magellanic Cloud have an apparent 

 visual magnitude of IS'O m. Thus the small Magellanic Cloud is at such a 

 distance that a star in it is 20'3 m. fainter than it would be if at a distance 

 of one parsec, from which it follows that the distance of this cloud is 10,000 

 parsecs. 



This example illustrates the utility of exact measurements of the light of 

 the stars. Much attention has been given of late years to Stellar Photometry. 

 In 1899 Prof. Pickering published the Revised Harvard Photometry giving 

 the magnitudes of all stars brighter than 65 m. In 1907 Messrs. iliiller and 

 Kempf completed a determination of 14,199 stars of the Northern Hemisphere 

 brighter than 7'5 m. In 1908 a catalogue of 36,682 stars fainter than 65 m. 

 was published at Harvard. These determinations derive additional importance 

 as they give the means of standardising estimates of magnitude made by eye, 

 particularly the many thousands of the Bonn Durchmusterung. 



By the labours of Prof. Pickering and liis colleagues at Harvard, Prof. 

 Schwarzschild, Prof. Parkhurst at Yerkes, Prof. Scares at Mount Wilson, and 

 others, the determinations of the magnitudes of stars by photography has made 

 rapid strides. As yet no complete catalogues of photographic magnitudes 

 corresponding to the Revised Harvard Photometry have been published, though 

 considerable parts of the sky and special areas such as the Pleiades have been 

 carefully studied. The determination of the photographic magnitudes 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 

 aod uniform scale has been to a large extent secured by the use of extrafocal 

 images, gratings, and screens in front of the object glass, and the study of the 

 effects of different apertures and different times of exposure. 



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 lO'O m. to 160 m. these agree very satis- 

 factorily. Near the limit at magnitude 20 m. there is naturally some dis- 

 cordance, as might be expected, but for the present this is not of great 

 importance. There is, however, a difference of 04 m. in the scale between 

 6-Om. and lOOni. which needs to be cleared up. t may remind you, to make 



