TEANSACTIONS OF THE SECTIONS. 



Section A ^MATHEMATICAL AND PHYSICAL SCIENCE. 

 Pkesident of the Section: — Sie F. W. Dyson, M.A., LL.D., F.K.S 



WEDNESDAY, SEPTEMBER 8. 



The President delivered the following Address :— 



Although at the present time our minds are largely absorbed by the war the 

 meeting of the British Association in Manchester indicates that we consider it 

 right to make our annual review of scientific progress. I shall therefore make 

 no apology for choosing the same eubject for my address as I should have 

 chosen in other circumstances. It is a subject far removed from war, being 

 an account of the manner in which astronomers have with telescopes and 

 spectroscopes investigated the skies and the conclusions they have reached on 

 what Herschel called 'The Construction of the Heavens.' 



Our knowledge of the fixed stars, as they were called by the old astronomers, 

 is of comparatively recent origin, and is derived from two sources : (1) the 

 measurement of small changes in the positions of the stars in the sky, and 

 (2) the analysis of the light received from them and the measurement of its 

 amount. The facts found for separate stars when arranged and classified give 

 us some insight into the structure of the Stellar Universe as a whole. The 

 discovery made by Galileo's telescope that the Milky Way consists of myriads 

 of stars may be taken as the beginning of Sidereal Astronomy. The further 

 study of the number of the stars and their general distribution has grown with 

 the light-grasping powers of the telescope, and in recent times has had the 

 assistance of photography. 



The changes of position of the stars among themselves are of two kinds. 

 The first consist of small periodic movements about a mean position due to 

 parallax, and in the case of double stars due to orbital motion. The observation 

 of these small angles has been made possible by the development of the telescope 

 for refined measurement. The most important contributions to this are the 

 Equatorial movement, the position micrometer, the heliometer, and the applica- 

 tion of photography. For the purpose of measurement the increase in focal 

 length and the perfect optical definition of the telescope are of greater import- 

 ance than increase in light-grasping power. 



The second class of movements, the proper motions of stars, are determined 

 by the comparison of the positions of the stars after an interval of years. 

 The accurate positions of stars in the sky are found by means of the transit- 

 circle and the astronomical clock. Both of these instruments have been slowly 

 brought to a high degree of perfection. The use of photography makes it 

 possible to extend the study of proper motions to the fainter stars. 



Accurate measures of the light of the stars have been in progress for the 

 last fifty years by the applications of photometers of various kinds to the 

 telescope. Many observations of magnitude, especially of variable stars, have 

 also been made by estimation, btit are brought to a uniform scale by com- 

 parison with photometric measures. In the last few years photography has 

 laeen very successfully applied to determine stellar magnitudes, and seems likely 

 to supersede visual observations to a large extent. 



