PRESIDENT S ADDRESS SECTION A. V 7 



deemed more satisfactory than the method of eye estimates in 

 common use; and he came to the conchision that the photographic 

 magnitudes of stars increase by equal areas for equal increases in 

 time of exposure, so proving that the photographic method of 

 dettrmininj,' star magnitudes proceeded on the same principle as 

 eye estimates, and anticipating by twenty-six years the same work 

 which has been gone over by several astronomers for the star 

 charting now in progress. -'' Professor Pritchard, however, came to 

 am ther conclusion, viz., that the area of the star image varies as 

 the square root of the time of exposure. 



The photo-heliogi*aph-*, which had been set up at Kew on the 

 earnest recommendation of Sir John Herschel, already referred to, 

 was completed at the end of February, and work on the sun was 

 begun with it on March 1st, 1858, but at first was not continuous 

 owing to the necessity for modifications in order to make the 

 exposure short enongh. This was ultimately accomplished by a 

 sliutler with a slit in it working in the focus of the objective. 



About 1860 Mr. De La Rue-' turned his attention to the possi- 

 bility of photographing the details of sun spots with his reilector, 

 and exhibited some on a scale of 3ft. to the sun's diameter. They 

 were not so good as he hoped to make them, but the cause he 

 thought was in the secondary magnifier. They were taken in one- 

 twentieth of a second. It does not appear thai they ever came to 

 perfection ; indeed it is well known now that the chief difficulty 

 is vibration in the atmosphere, which is seldom absent ; but he 

 pursued the subject, and we are told in 186.3-*^ that he had exhibited 

 some photographs of sun spots on the enormous scale of 13ft. for 

 the sun's diameter, and also some prints from them produced by 

 Herr Pretsche's process (untouched by the graver). 



Meantime this enthusiast, whose ability and energy for many 

 years led the way in the application of photography to astronomy, 

 was busy photographing star clusters with his reflector, but he found 

 it better to use a large portrait lens, which gave very encouraging 

 results. He remarks " the difficulty does not consist in fixing the 

 images of the stars, but in finding the images when they are 

 imprinted, for they are no bigger than the species common to the 

 best collodion." 



At this time-^ some curiosity existed as to the possibility of 

 photographing comets. " I tried" writes De La Rue, " w ith my re- 

 flector, on the appearance of Donati's comet in 1858, several times, 

 without success, and on the appearance of the comet of the 

 present year (1861) I tried not only with my telescope, but also 

 with a portrait lens, and with an exposure of fifteen minutes, not 

 seconds, but I failed to get the slighest trace with either." Tlie 

 care in stating the time of exposure was probably due to a report 

 that Mr. Usherwood'", of Walton Common, in Surrey, had secured 

 a photograph of Donati s comet on September 26th, 1858. He 

 used an ordinary portrait lens without equatorial stand, but set 



