ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 305 



comparable one with another : nevertheless some important generalisations 

 were arrived at. No substance which could be conveniently employed 

 for prisms and lenses was found to surpass quartz in diactinicity. 



(2) ' The absorption of the invisible rays by reflexion from polished 

 surfaces.' 



A small polished plate of the substance under experiment was supported 

 at an angle of 45° opposite the vertical slit of the apparatus, and the 

 source of the rays was arranged so that they should be reflected in the 

 direction of the axis of the tube. ' It was found that no judgment of 

 the perfection of the reflecting power could be formed from the colour of 

 the metal.' For example, gold possesses the power of reflecting all the 

 rays, even the most refrangible, very equally, though somewhat feebly. 

 Next to gold ranks burnished lead, some parts of the spectrum reflected 

 from lead being more intense than that from gold. The spectrum 

 reflected from these two metals was found to be longer than that obtained 

 by reflection from any other metallic surface examined. The spectrum 

 reflected from a silver surface was characterised by a sudden cessation for 

 a certain distance of the image on the photographic plate ; that is to say, 

 in a certain portion of the spectrum the rays had been absorbed, some 

 more refrangible being transmitted. The reflection from steel was more 

 intense than that from any other surface employed, with, perhaps, the 

 exception of tin. 



Speculum-metal, platinum, zinc, aluminium, mercury, cadmium, 

 copper, and brass were also examined. 



The foregoing experiments on reflection from metallic surfaces were 

 undertaken in consequence of the difficulty experienced in obtaining a 

 spectrum, all parts of which were even approximately in focus in the 

 same plane. The results were not, however, considered favourable to the 

 substitution of a speculum for a lens. 



(3) Photographic effects of the electric spectra of different metals taken 

 in air. 



(a) Pure Metals. Although each metal was found to have a distinc- 

 tive spectrum, as in the case of the ordinarily visible rays, yet it is re- 

 markable that no important diSerence is apparent in the less refrangible 

 end. The photographic lines of the air-spectrum are most marked in the 

 less refrangible portion, whilst the characteristic lines of the metals are 

 particularly evident in the more refrangible parts. The more volatile 

 metals gave the most intense spectra — those of bismuth, antimony, 

 cadmium, zinc, and magnesium being the most prominent in this respect. 

 A certain similarity was observed in the spectra of allied metals; this was 

 the case with the three last-named metals, and also in the case of iron, 

 cobalt, and nickel, and with bismuth and antimony, as well as with chro- 

 mium and manganese. In consequence of imperfections in the methods of 

 experimenting, ihe true relative length of the spectra was not accurately 

 determined. 



(b) Spectra of Alloys. Dr. Miller states that 'When equal weights 

 of two metals are employed (tin and lead, for example, or cadmium and 

 lead) a compound spectrum exhibiting the lines due to both metals is 

 produced ; and it is not always the more volatile metal that predomi- 

 nates.' 



1880. X 



