41: 



Professor H. H. Turner 



[Jan. 8L 



angular size (viz. about y^otti part of the distance in each case), we 

 know that the sun must be about a milhon miles in diameter and 

 the moon only about 2500 miles, using round numbers throughout 

 for simplicity. 



Thus the sun is' a veritable giant compared with the moon, in 

 spite of the similar apparent size of the discs ; but this we discover 

 only when we have measured the distances. 



The sun and moon present to us large discs which we can study 

 in detail, and the study of the disc of the sun by means of the 

 spectroheliograph has had new triumphs which, owing to the War, 

 have not yet been seen in this room, so that I may be pardoned for 

 exhibiting one. 



(Mount Wilson spectroheliogram shown.) 



But when we come to the stars there is no disc. If one seems to 

 see discs for these objects, the appearance comes through the imper- 

 fections of the telescope. Hence it would seem to be superfluous to 

 inquire about their distance. When Mark Twain had been roughly 

 handled at Niagara Falls, and the doctor reported that only sixteen 

 of his wounds were mortal, he said " he did not care about the 

 others." In the same way we might argue that since the stars have 

 no appreciable discs we need not care about their distances. 



That, however, was not the attitude of men of science. Tney 

 went to work to measure their distances, and though the difficulties 

 were heart-breaking they were attacked and overcome. Here is a 

 table showing a sensible fraction of the life-work of an eminent 

 Scotsman, Sir David Gill, observing at the Cape of Good Hope. It 



includes the famous a Centauri, the first star to have its distance 

 measured at all, which again was by a Scotsman, Henderson, also 

 observing at tlie Cape of Good Hope. 



Sir David Gill was accustomed to describe the difficulties of 

 determining that distance by saying that it was like trying to 



