170 Britain's Heritage of Science 



the red flames shooting out from the edge of the sun were 

 clearly shown in his photographs. This was an important 

 achievement, as there had been some doubt whether these 

 so-called protuberances were real phenomena belonging to 

 the sun. De la Rue also introduced the daily photographic 

 record of the sun, originally carried out at Kew, and now 

 at Greenwich and other places in the British Empire. 



So far all concave mirrors used in reflecting telescopes 

 had been made of speculum metal, an alloy of tin and 

 copper, which tarnishes in the course of time. A process of 

 polishing almost as troublesome as the original shaping of 

 the surface had then to be undertaken. It was, therefore, 

 a substantial step in advance when Andrew Ainslie Common 

 (1841-1903), an engineer by profession, introduced mirrors 

 made of glass silvered at the surface, for the silvering could 

 be renewed without interfering with the shape of the surface. 

 Common acquired great skill in grinding the surfaces of glass ; 

 one of his mirrors, three feet in diameter, is now at work at 

 the Lick Observatory, and a five -foot mirror forms part of 

 the equipment of Harvard. The photograph which Common 

 obtained of the nebulae in Orion first showed the complicated 

 structure of that wonderful object, and was described by 

 Sir William Abney as " epoch-making in astronomical 

 photography." 



With the introduction of dry plates a new era began for 

 Astronomy, and one of the most persevering and successful 

 workers in the field was Isaac Roberts (1829-1904), whose 

 beautiful collection of photographs of celestial objects, and 

 notably of nebulae, form a permanent record which will in 

 the future prove of the greatest value. Roberts was a builder 

 by profession. In 1890, the year after his retirement from 

 business, he moved from Liverpool to Crowborough, in 

 Sussex, where the clear air allowed him to produce his 

 finest work. 



Until the middle of last century the astronomer was 

 confined in his observations to the use of the telescope ; he 

 could determine the position of stars,investigate their displace- 

 ments in the sky, and examine the structure of star clusters 

 and nebulae. Beyond this he was unable to go, until the 

 invention of the spectroscope gave him the power to extend 



