Absorption Spectra. . 225 



absorption bands is known as an absorption spectrum. 

 Thus, in the accompanying spectrum of blood (Plate I, Fig. 4) 

 produced by allowing white light to pass through a thin 

 layer of blood, we see two dark bands, one situated in the 

 yellow and the other in the green, due to this special solution 

 absorbing, or blotting out, the particular rays, which, if no 

 coloured medium were there, would have appeared as yellow 

 and green rays respectively. Such a spectrum is held, so far 

 as spectroscopic analysis is concerned, as characteristic of 

 the presence of oxyhaemoglobin, the colouring matter of 

 arterial blood, and its essential features are the position and 

 character of its absorption bands. The same remark 

 applies to absorption spectra generally. So far as spectro- 

 scopic examination is concerned their analytic value is 

 essentially dependent on the character and position of their 

 absorption bands. Keeping this fact in view, the query 

 " Are these characters constant for the same substances?" 

 becomes a very important one. The variation in the nature 

 of the light transmitted by a thick and a thinner film of 

 gold will have prepared us for the answer, but unfortunately 

 not for the whole cause of variation in the absorption spectra 

 of the same bodies. Thus, Herr Vogel has shown {Nature, 

 Vol. XIX., p. 495) that in some recorded instances the 

 spectrum in the solid condition differs from that of the 

 same substance in the liquid condition. That variation in 

 the dissolving medium may, or may not, produce variation 

 in the resulting spectra. Again, bands occupying exactly 

 the same position were shown to characterise different 

 bodies. After the above, which is merely a very brief 

 epitome of some of this observer's strictures, a full account 

 of which appears in the monthly report of the Berlin 

 Academy of Sciences, the author naturally points out the 

 importance of not placing too much reliance on spectroscopic 

 analysis pure and simple, and lays stress on the advantage 

 to be derived by observation of the spectra of media acted 



