94 THE KINGDOM OF MAN 



astronomers as a curiosity. Visual observations were 

 begun at Greenwich in 1875, but were found to be affected 

 by instrumental errors. The introduction of dry plates, 

 and their application by Vogel in 1887, was the be- 

 ginning of general use of the method, and line-of-sight 

 work is now a vast department of astronomical industry. 

 Among other by-products of the method are the ' spec- 

 troscopic doubles,' stars which we know to be double, 

 and of which we can determine the period of revolution, 

 though we cannot separate them visually by the greatest 

 telescope. 



Work on the sun has been entirely revolutionised 

 by the use of photography. The last decade has seen 

 the invention of the spectro-heliograph which simply 

 means that astronomers can now study in detail portions 

 of the sun of which they could previously only get a bare 

 indication. 



More of the same story could be related, but enough 

 has been said to show how full of life and progress is 

 this most ancient and imposing of all sciences. 



A minor though very important influence in the 

 progress of astronomy has been the provision, by the 

 expenditure of great wealth in America, of great tele- 

 scopes and equipments. 



In 1877 Sir George Darwin started a line of mathe- 

 matical research which has been very fruitful and is of great 

 future promise for astronomy. As recently as last April, 

 at the Royal Astronomical Society, two important papers 

 were read one by Mr. Cowell and the other by Mr. 

 Stratton which have their roots in Sir George Darwin's 

 work. The former was led to suggest that the day is 

 lengthening ten times as rapidly as had been supposed, 

 and the latter showed that in all probability the planets 

 had all turned upside down since their birth. 



