cvi. PRESIDENT'S ADDRESS. 



mind, especially when he comes to speak of the parallaxes and 

 proper motions of stars. 



The parallax, or the angle which the radius of the earth's orbit 

 subtends at the star, is in most cases too minute for practical 

 measurement, and in all exceedingly small. Many of the stars 

 have perceptible motions of their own, and it has been shown, 

 and recently independently confirmed, that at least for extensive 

 parts of space there are a nearly equal number of stars moving in 

 exactly opposite directions. By other methods we find the sun's 

 motion in space to be about 1 9 kilometres per second, and it follows 

 that, instead of the already immense base line of the diameter of 

 the earth's orbit, we can utilise a base line of still more unlimited 

 length by observing the positions of the stars after any interval 

 of time, allowing a length of 19 kilometres for each second. In 

 consequence of the proper motions of the stars, this method 

 cannot be applied to any particular star alone, but by taking a 

 small group of perhaps 20 or more, of the same magnitude and 

 proper motion, the average parallax obtained appears to be 

 approximately correct, the motions of the stars composing the 

 group being in different directions and neutralising each other 

 for the purpose of this calculation. Further developments as to 

 the distribution in space of stars of different magnitude and 

 others follow from these results, but into these I must not 

 attempt to tread. The result of the photographs of the asteroid 

 Eros in 1900-1, lately communicated, gives the sun's parallax 

 within 2 i,oooths of a second of arc of the accepted amount. In 

 1931 Eros will be much more favourably situated for this purpose, 

 and it is thought that the sun's parallax should then be estimated 

 within i io,oooth of its actual value. At the recent transit of 

 Mercury many observers noticed the nebulous ring surrounding 

 the planet, which has suggested the presence of an atmosphere. 

 At Bourges the bright spot on the disc was also noticed. Of 

 this, if a real phenomenon, I believe no satisfactory explanation 

 has yet been given. During the opposition of Mars in 1907, 

 when he appeared so brilliant to the naked eye, a long series of 

 photographs was taken at the Lowell Observatory, and also by 



