SIR. WILLIAM HUGGINS CAMPBELL. 311 



draining bacli from the positions in wliicli the plates had often to be put, that 

 we did not persevere in our attempts to photograph the stellar spectra. I 

 resumed them with success in 1875, as we shall see further on. 



Whenever the nights were fine, our work on the spectra of the stars went 

 on, and the results were communicated to the Royal Society in April. 1864; 

 after which Dr. Miller had not sufficient leisure to continue working with me. 



* * * I pass on at once, therefore, to the year 187G, in which, by the 

 aid of the new dry plates, with gelatin films, introduced by Mr. Kennett, 

 I was able to take up again, and this time with success, the photography 

 of the spectra of the stars, of my early attempts at which I have already 

 spoken. 



By this time I had the great happiness of having secured an able and enthu- 

 siastic assistant by my marriage in 1875. 



The great and notable advances in astronomical methods and discoveries 

 by means of photography, since 1875, are due almost entirely to the great ad- 

 vantages which the gelatin dry plate possesses for use in the observatory over 

 the process of Daguerre, and even over that of wet collodion. The silver- 

 bromide gelatin plate, which I was the first, I believe, to use for photograph- 

 ing the spectra of stars, except for its grained texture, meets the need of the 

 astronomer at all points. This plate possesses extreme sensitiveness ; it is 

 always ready for use ; it can be placed in any position ; it can be exposed for 

 hours; lastly, immediate development is not necessary, and for this reason, 

 as I soon found to be necessary in tliis climate, it can be exposed again to 

 the same object on succeeding nights ; and so make up by successive install- 

 ments, as the weather may permit, the total long exposure which may be 

 needful. 



The power of the eye falls off as the spectrum extends beyond the blue, 

 and soon fails altogether. There is, therefore, no drawback to the use of 

 glass for the prisms and lenses of a visual spectroscope. But while the sen- 

 sitiveness of a photographic plate is not similarly limited, glass, like the eye, 

 is imperfectly transparent, and soon becomes opaque, to the parts of the spec- 

 trum at a short distance beyond the limit of the visible spectrum. To obtain, 

 therefore, upon the plate a spectrum complete at the blue end of stellar light, 

 it was necessary to avoid glass and to employ instead Iceland spar and rock 

 crystal, which are transparent up to the limit of the ultra-violet light which 

 can reach us through our atmosphere. Such a spectroscope was constructed 

 and fixed with its slit at the focus of the great speculum of the Cassegrain 

 telescope. 



How was the image of a star to be easily brought, and then kept, for an hour, 

 or even for many hours, precisely at one place on a slit so narrow as about the 

 one two-hundredth of an inch? For this purpose the very convenient device 

 was adopted of making the slit-plates of highly polished metal, so as to form a 

 divided mirror, in which the reflected image of a star could be observed from 

 the eye end of the telescope by means of a small telescope fixed within the 

 central hole of the great mirror. A photograph of the si^ectrum of a Lyrse, 

 taken with this instrument, was shown at the Koyal Society in 187G. 



In the spectra of such stars as Sirius and Vega, there came out in the ultra- 

 violet region, which up to that time had remained unexplored, the completion of 

 a grand rhythmical group of strong dark lines, of which the well-known hydro- 

 gen lines in the visible region form the lower members. Terrestrial chemistry 

 became enriched with a more complete knowledge of the spectrum of hydrogen 

 from the stars. Shortly afterwards, Coruu succeeded in photographing a 

 similar spectrum in his laboratory from earthly hydrogen. 



