1862.] 167 



at a lecture given at the Royal Institution in 1853; but the 

 author, for reasons he mentioned, did not then further pursue the 

 subject. Having subsequently found that the spark of an induction- 

 coil with a Ley den jar in connexion with the secondary terminals 

 yielded a spectrum quite bright enough to work by, he resumed 

 the investigation, and examined the spectra exhibited by a variety of 

 metals as electrodes, as well as the mode of absorption of the rays 

 of high refrangibility by various substances. The spectra of the 

 metals may be viewed at pleasure by means of fluorescence, and the 

 mode of absorption of the invisible rays by a given solution may be 

 at once observed ; but there are difficulties attending the preparation 

 in this way of sufficiently accurate maps of the metallic lines ; and 

 the great liability of the rays of high refrangibility to be absorbed 

 by impurities present in very minute quantity renders the certain 

 determination of the optical character, in this respect, of substances 

 which are only moderately opaque a matter of considerable difficulty. 

 Having found that Dr. Miller had been engaged independently at 

 the same subject, working by photography, the author deemed it 

 unnecessary to attempt a delineation of the metallic lines (for which, 

 however, he has recently devised a practical method that was found 

 to work satisfactorily), or to examine further the absorption of rays 

 of high refrangibility by solutions of metallic salts, &c. 



The present paper contains therefore mainly results obtained in 

 other directions in the same wide field of research. Among the 

 metals examined, the author had found aluminium the richest in 

 invisible rays of extreme refrangibility ; and accordingly aluminium 

 electrodes were employed when the deportment of such rays had to 

 be specially examined. As the bright aluminium lines of high re- 

 frangibility do not appear to have been taken by photography, a 

 drawing of the aluminium spectrum is given, with zinc and cadmium 

 for comparison. 



The author has also described and figured the mode of absorption 

 of the invisible rays by solutions of various alkaloids and glucosides. 

 Bodies of these classes, he finds, are usually intensely opaque, acting on 

 the invisible spectrum with an intensity comparable to that with which 

 colouring matters act on the visible. This intensity of action causes 

 the effect of minute impurities to disappear, and thereby increases 

 the value of the characters observed. It very often happens that at 



N 2 



