47 



same time, a very precise determination has been made of 

 the constant of that " aberration of light " which was 

 discovered by Bradley. 



The remaining small telescope is the Photoheliograph, 

 also a photographic telescope, made by DALLMEYER AND 

 SON, for the Transit of Venus Expedition to New Zealand in 

 1874. Its aperture is 4 inches, usually stopped down to 2-9, 

 and it gives an image of the sun of 8 inches diameter on the 

 photographic plate. With this photoheliograph, or with 

 others of similar construction, photographs of the sun have 

 been regularly taken, day by day, whenever possible, since 

 April 1874, and the series obtained at Greenwich has been 

 supplemented by similar photographs taken at the Royal 

 Observatory, Cape of Good Hope, and at Observatories in 

 India and in Mauritius. In this way a continuous record 

 has been secured of the changes in the surface of the sun 

 through four complete n-year cycles of sunspot activity. 

 The year 1913 proved memorable as the one in which the 

 sun's surface was least disturbed for a whole century, while 

 the month of August 1917 showed the greatest extent of 

 sun-spots since the Greenwich record began. From these 

 photographs the rotation periods of the sun for different 

 latitudes have been determined, and also the position of the 

 sun's axis. Also a curious relation has been detected between 

 the area of a spot and its position east and west of the central 

 meridian of the disc, groups being both more numerous 

 and larger before reaching the central meridian than after- 

 wards. Another relation between the sun and the earth 

 has also been established in that disturbances of the earth's 

 magnetism tend to recur with the return of a meridian of 

 the sun to the centre of the disc, proving that such disturbances 

 were due to the impact of streams of particles driven off 

 from restricted areas of the sun's surface. 



This discovery was one of the fruits of the establishment 

 by Airy of a Magnetical Department. Initiated almost at 

 the beginning of his administration he greatly increased its 

 efficiency by enlisting the aid of photography, then in its 

 infancy, for the registration of the variations of terrestrial 

 magnetism. Greenwich, therefore, possesses a magnetic 

 record by photography continuous from 1848 onward, and, 

 in 1822, Christie commenced the publication in the annual 

 Greenwich Volumes of reproductions of the photographic 

 traces, for the days of greatest disturbance, and it was these 

 reproductions which led directly to the discovery that the 

 disturbances so frequently occurred with the return of the 



