1880.J Dr. W. Uufjtjins on PliotngrapMc Spcdrn of the Stars. 285 



WEEKLY EVENING MEETING, 



Friday, February 6, 1880. 



Warren De La Rue, Esq., M.A. D.C.L. F.R.S. Secretary and 

 Vice-President, in the Chair. 



William Hugqins, D.C.L. LL.D. F.R.S. M.B.L 



The Photographic Spectra of the Stars, 



In the year 1863 my friend Dr. William Allen Miller exhibited on 

 the screen in this room a photograph of the spectrum of the star 

 Sirius, which we had taken the evening before in my observatory. 

 The images of stars in the telescope had already been photographed 

 as points, but this was the first time that their rays after dispersion 

 by a prism had recorded themselves upon a photographic plate. For 

 certain instrumental reasons, the photographs which we then took 

 did not possess sufficient purity of the spectrum to give them a 

 scientific value. 



Several researches in other directions to which I subsequently 

 devoted myself, prevented me for some years from resuming this 

 inquiry, until a few years ago, when I took up the subject again. I 

 purpose this evening to give an account of this recent work, and of 

 the results which have come out of it. 



Our common notion of light is limited not by the actual extent of 

 range of the radiations of a luminous body, but by the power of our 

 eyes to see them. Of the long range of radiations which comes from 

 highly heated matter, the sun for example, only a small portion falls 

 within the power of the eye. Beyond the extreme violet, where 

 visibility ends, a great range of shorter vibrations beats upon the eye, 

 and we know it not. So on the other side below the red all con- 

 sciousness of light fails us ; but here another sense, that of the feeling 

 of heat and warmth, enables us still to know that a radiated influence 

 from the hot body is coming upon us. These two invisibles, the 

 ultra-violet and the ultra-red, though they cannot stimulate our eyes 

 directly, can make themselves known to us mediately, through certain 

 actions on other bodies. 



One of these is the disturbing influence they exert on delicately 

 balanced salts of silver, which we call their photographic power. 

 This action was regarded as so exclusively the property of the ultra- 

 violet portion of the spectrum, that these rays have been distinguished 

 by the names, " chemical rays," " photographic rays." Quite recently. 



