298 REPOET— 1880. 



advance of science lias constantly led to the separation of phenomena 

 which were formerly considered to be connected together, and we believe 

 that the further development of the different points we have attempted 

 to discuss, in which different observers have strongly taken up opposite 

 opinions, will lead to the blending together of different views rather than 

 the entire elimination of one of them. 



§ 3. Emission Spectra of Rats more Refrangible than H. Report hy 

 W. N". Hartley, Professor of Chemistry, Eoyal College of Science for 

 Ireland, Dublin. 



Schule was the first to show that silver chloride when exposed to light 

 transmitted through a prism was blackened more by the violet rays than 

 by those of any other colour.^ 



Wollaston who, prior to Fraunhofer, perceived certain obscure rays in 

 the Solar Spectrum, repeated the experiments of Schule, and found that 

 the blackening of silver chloride extended not only over the surface occu- 

 pied by the violet rays, but also to an equal degree over an equally large 

 surface beyond the visible spectrum. ^ Shortly after the discovery of 

 photography. Sir John Herschel resumed this subject, and remarked that 

 different sensitive substances when exposed to the action of the spectrum 

 behave very unequally. The maximum of chemical action takes place 

 sometimes in one colour, sometimes in another, sometimes even outside 

 the spectrum, and it was always observed to extend beyond the violet. 

 Herschel unsuccessfully endeavoured to ascertain whether there are inac- 

 tive spaces in the chemical spectrum, by exposing sensitised paper pre- 

 pared according to the process of Mr. Fox Talbot.^ 



He showed at this time that the ultra-violet rays are not completely 

 invisible. They produce upon, the eye a sensation which is not that of 

 violet nor of any other prismatic hue, but rather resembling what one 

 might call a lavender-grey tint. He proposed to apply the name lavender 

 to the obscure rays which produce the tint in question, in order to ab- 

 breviate the awkwardly sounding expression ultra-violet rays, and to 

 avoid the ambiguity attached to the term chemical rays, which in point of 

 fact are found in all parts of the spectrum. 



M. Edmond Becquerel was more fortunate in demonsti-ating the exist- 

 ence of chemically inactive rays.* He showed impressions on prepared 

 paper and on plates of iodised silver, indicating maxima and minima of 

 chemical action. Becquerel gave the name of jjhosphorogenic spectrum 

 to the collection of rays which show the phenomenon of phosphorescence. 

 This spectrum extends beyond the violet, and consists of rays identical 

 with the luminous and the chemical rays. We are indebted to Prof. 

 Stokes for a means of studying the extremely refrangible rays by means 

 of fluorescence.® 



If a spectrum be projected on to certain substances such as quinine 

 sulphate, tincture of turmeric, or glass coloured with uranic oxide, these 

 substances become luminous, for a considerable distance beyond the 

 region of the violet rays, and the rays thus rendered visible are always 



' Traite Chemique de VAir et du Feu, sec. 66, 1781. 

 = Pm. Trans. 1802. " Ibid. 18 tO. 



* Bihliothcqne Uidverselle de Geneve, t. xl. 1842. 

 5 Fhil. Trans. 1852. 



