PROGKESS IN ASTRONOMY. 131 



together that they appear as one if the plane of motion passes near 

 the earth. A line common to the spectra of both stars will appear 

 double, twice in each revolution, when the motion to or from the earth, 

 or, as it is termed, "in the line of sight," is greatest. " Spectroscopic 

 doubles," as these stars are called, yield up many of their secrets 

 Avhich otherwise would elude us. Their time of revolution, the size 

 of the orbit, and the combined mass can b'fe determined. 



To return from the stars to the sun. 



By the device of throwing an image of the sun on the slit of tne 

 spectroscope the spectra of solar spots have been studied froai 1866 

 onward, and a little later the brighter portions of the sun's outer 

 envelopes, revealed till then only during eclipses, were brought within 

 our ken spectroscopically so that they are now studied every day. 



CELESTIAL PHOTOGRAPHY. 



Wedgewood and Davy, in 1802, made prints on paper b}' means of 

 silver salts, but it was not until 1830 that Niepce and Daguerre founded 

 photography, which Arago, in an address to the French Chamber, at 

 once suggested might subsequently be used to record the positions of 

 stars. 



In 1830 we find Sir John Herschel carrying out a series of experi- 

 ments so important for our correct knowledge of the sequence of steps 

 in the early stages of photograph}^ that I have no hesitation in quoting 

 from one of Herschel's manuscripts relating to a deposit on a glass 

 plate of " muriate" [chloride] of silver from a mixed solution of the 

 nitrate with common salt. The manuscript states: "After forty -eight 

 hours [the choride] had formed a film firm enough to bear draining 

 the water off very slowly b}- a syphon. Having dried it, I found that 

 it was very little affected by light, and by washing it with nitrate of 

 silver, weak, and drying it, it became highly sensitive. In this state 

 I took a camera picture of the telescope on it." 



The original of the above-mentioned photograph, the first photo- 

 graph ever taken on glass, is now in the science collection at the Vic- 

 toria and Albert Museum, South Kensington. 



In the earl}^ days of photography colored glasses were first used to 

 investigate the action of different colors on the photographic plate. 

 Sir John Herschel was among the first to propose that such investi- 

 gations should be made direct with a spectrum, and he, like Dr. J. W. 

 Draper, stated that he had found a new kind of light l)eyond the blue 

 end of the spectrum, as the photographic plate showed a portion of the 

 spectrum there which was not visible to the eye. Advance followed 

 advance and in 1812 Becquerel photographed the whole solar spec- 

 trum, in colors, with nearly all the lines registered by the hand and 

 eye of Fraunhofer, not only the blue end, but the complete spectrum 



