president's address SECTION A. 79 



it impossible to get with this instrument, intended for vision, 

 such perfect photographs as he desired, he tried first a reflector of 

 13in., but ultimately gave it up, and determined to make an 

 1 l|-in. objective corrected for photographic purposes. This was 

 not accomplished until December, 1864, and he did not get a 

 satisfactory negative until March 6th, 1865. The construction 

 of this lens was difficult, because its progress could not be tested 

 by the visual image. Mr. Rutherford got over the difficulty by 

 testing it with a spectroscope. With this instrument stars down 

 to ninth magnitude were taken with three minutes' exposure, and 

 the only photograph of the moon taken with it was sharper than 

 any other Mr. Rutherford had ever seen. 



It was suggested at the time that photographs of the sky 2° on a 

 side might be taken with it.^** 



The power to obtain photographs of stars down to the ninth 

 magnitude with such a small aperture and an exposure of three 

 minutes promises to develop and increase the application of photo- 

 graphy to the mappinu; of the heavens, and in some measure to 

 realise the hopes that have so long been deferred and disappointed. 



On January 11th, 1869, M. Janssen^' presented to the Academie 

 of Sciences a short note pointing out that it was possible to isolate 

 any part of a spectrum bj- placing a second slit near the eyepiece 

 — an idea which underlies some of the most remarkable results of 

 the present day, but it lay dormant until 1892. 



In 1871 Dr. Diaper^** completed a 28in. silvcred-glass reflector, 

 made for the purpose of photographing star sj^ectra, and in May, 

 1872, and again in August, he photographed the spectrum of 

 Vega, showing four strong lines. Dr. Huggins, as we have seen, 

 photographed the sjjectrum of Sirius on a collodion plate in 1863. 

 In 1870^^ Professor C. A. Young succeeded in photographing the 

 prominences of the sun. Negatives were made showing the solar 

 disc on a scale 2in. in diameter, Avhich represented clearly the 

 general form of the prominences, but the telescope was too small 

 for good definition, and the work was given up. The light of the 

 hydrogen line Y was used because more actinic than K. They 

 were taken with an open slit on the spectroscope. 



In 1872 Mr. Ellery photographed the moon with the great 

 reflector at Melbourne with marked success, and produced the 

 finest photographs that had been seen up to that time. 



In 1873-4 many persons urged that photography should be 

 applied to the transit of Venus, and Sir G. B. Airy, after some 

 hesitation, adopted this as an auxiliary method, and in 1874 it was 

 used by the majority of parties sent out as a means of determining 

 the position of Venus on the sun. It did not prove so successful 

 as it was hoped it would, but on many of the photographs taken 

 in New South Wales the ring of light surrounding the planet at 

 and near the sun's limb was clearly recorded and shown to be 

 brighter than the sun itself by the greater deposit of silver which 



