226 



Profs. Liveing and Dewar. 



[May 19, 



collimator, each, witli an aperture of 1^- inch and focal length 18 

 inches, the lenses being of quartz, cut perpendicularly to the axis and 

 unachromatised, giving a very good definition with monochromatic 

 light. The chromatic aberration is in this case an advantage, for 

 when the telescope is in focus for hues 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 refrangi- 

 ble rays that of the fourth order. The source of light was the electric 

 arc taken in a crucible of magnesia or lime ; 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 the eye- 

 piece, the second metal was next introduced, 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 hereto- 

 fore supposed coincident with it (wave-length 610T9), 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 for a short time to be reversed, the other line remaining 

 narrow and quite unaltered. When the lithium had evaporated, and 

 both lines were again narrow, a small piece of Iceland spar was dropped 

 into the crucible, which immediately caused the expansion, and on one 

 occasion the reversal, of the more refrangible line, while now the less 

 refrangible line was unaffected. 



In this way we satisfied ourselves that the calcium line is the more 

 refrangible of the two, and is probably represented by the line at wave- 

 length 6101 '9 in Angstrom's normal solar spectrum, while the lithium 

 line appears to be unrepresented. 



In the case of iron, which gives such a multitude of lines, it was 

 a priori probable that some lines would be coincident, or nearly so, 

 with lines of other elements ; and in fact we find that in five-sixths of 



