1881.] on the Oihjin and Llentiti/ of Spectra, 701 



70 lines given in Angstrcim's map as coninion to two or more sub- 

 stances, liiis found tliiit 56 are double or treble, 7 more doubtful, and 

 only 7 appear definitely singb;, and lie remarks:* "The complete 

 investigation of the matter re([uires that the bright line spectra of 

 the metals in question should be confronted with each other and with 

 the solar spectrum under enormous dispersive power, in order that 

 we may determine which of the components of each double lino 

 belongs to one and which to the other element." It is this confronting 

 of the bright line spectra of some of the terrestrial elements which 

 we have attempted, and of which we now give an account. For the 

 dispersion we have used a reflecting grating similar to that used by 

 Young, with 17,290 lines to the inch, and a ruled surface of about 31 

 square inches ; telescope and collimator, each with an aperture of 

 l.V inch and focal length 18 inches, the lenses being of quartz, cut 

 perpendicularly to the axis and unachromatised, giving a very good 

 detinition with monochromatic light. The chromatic aberration is in 

 this case an advantage, for when the telescope is in focus for lines in 

 the spectrum of any given order, the overlapping parts of spectra of 

 different orders are out of focus, and their brightness consequently 

 more or less enfeebled. We have sometimes used green or blue 

 glasses to enhance this result. The telescope and the collimator 

 were generally fixed at about 45°, the collimator being more nearly 

 normal to the grating than the telescope, and the grating moved to 

 bring in successive parts of the spectra. For the parts of the spectra 

 less refrangible than the Fraunhofer line E the spectrum of the third 

 order was employed, for the more refrangible rays that of the fourth 

 order. The source of light was the electric arc taken in a crucible 

 of magnesia or lime, the image of the arc being focussed on the slit ; 

 and, for the examination of any supposed coincidence, first one metal 

 was introduced into the crucible, and the line to be observed placed 

 on the pointer of tlie eye-piece ; the second metal was then intro- 

 duced, and then in most cases, as detailed below, two lines were seen 

 where only one was visible before, and the pointer indicated which of 

 the two belonged to the metal first introduced. In some cases where 

 both metals were already in the crucible, we had to reinforce the 

 spectrum of one of the metals by the introduction of more of that 

 metal, which generally brought out the spectrum of that metal more 

 markedly than the other, and enabled us to distinguish the lines with 

 a high degree of probability. Thus the crucibles of magnesia, or 

 the carbons, always contain sufficient lithium to show the orange line 

 and the calcium line heretofore supposed coincident with it (wave- 

 length 6101-9), but we observed these lines quite distinct and 

 separated by a distance, estimated by the eye in comparison with the 

 distance of neighbouring titanium lines, at about one division of 

 Angstrom's scale. On dropping a minute piece of lithium carbonate 

 into the crucible, the less refrangible line was seen to expand and 



* ' American Journal of Science,' vul. xx. p. 353. 



