132 PROGRESS IN ASTRONOMY. 



from Draper's "latent light," as he called the ultra violet rays, to tJie 

 extreme red end. 



The first photograph of a celestial object was one of the moon 

 secured by Dr. J. W. Draper in 1840 ; we had to wait until 1845, so 

 far as I know, before a daguerreotype was taken of the sun ; this was 

 done by Foucault and Fizeau, while the first photograph of a star — 

 Vega — was taken at Harvard in 1850. After the introduction of the 

 wet collodion process regular photographs of the sun's surface were 

 commenced, at Sir John HerscheFs recommendation, at Kew, in 1858, 

 and the total solar eclipse of 1860 was made memorable by the photo- 

 graphs of De La Rue, who before that time had secured most admi- 

 rable photographs of the moon, as also had Rutherfurd. 



Photography now began to pay the debt she owed to spectrum 

 analysis. 



The first laboratory photograph of the spectra of the chemical ele- 

 ments was taken by Dr. W. A. Miller in 1862. 



Rutherfurd was the first to secure a photograph of the solar spec- 

 trum with considerable dispersion by means of prisms. 



In 1863 Mascart undertook a complete photographic investigation 

 of the ultra violet portion of the solar spectrum, a work of no mean 

 magnitude. He, however, did not employ a train of prisms for pro- 

 ducing the spectrum, but a detraction grating, using the light reflected 

 from the first surface. The first photograpn of the spectrum of a star 

 was secured by Henry Draper, the son of Dr. J. W. Draper, one of 

 the pioneers in photography, in 18Y2. 



It was not till the introduction of dry plates in 1876 that the pho- 

 tography of the fainter celestial objects or of their spectra was possi- 

 ble, as a long exposure was naturally required. Stellar spectra were 

 photographed by Huggins in 1879 and in the next year Draper photo- 

 graphed the nebula of Orion. As the dry plates became more rapid 

 and as longer exposures were employed, revelation followed revela- 

 tion ; the nebula? as seen by the naked eye, and even some stars, were 

 found by the Henrys, Roberts, Max Wolf, Barnard, and others to be 

 but the brighter kernels of large nebulous patches. 



This new application of photography, depending upon long expo- 

 sures (the longest one I know of has extended to forty hours), had an 

 important reflex action on the mechanical parts of the telescope; it 

 was not only necessary to keep the faintest star exactly on the same 

 part of the plate during the whole of the exposure, but night after 

 night the stellar image must be brought onto the same part of the 

 plate so that the exposure might be continued. 



A sj'stem of electric control of the going of the driving clock of the 

 telescope by means of a sidereal clock was introduced, the simplest 

 one being designed by Russell of Sydney, a most elaborate one by 

 Grubb of Dublin. 



