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XXV. — A SIMPLE METHOD OF OBSERYING FAINT LINES 

 WITH DIFFRACTION SPECTEOSCOPES. By W. N. 

 HARTLEY, F.R.S.E., &c., Professor of Chemistry, Royal 

 College of Science, Ireland. 



[Read, January 21, 1884.] 



In observing spark spectra with diffraction gratings there is gene- 

 rally much difficulty attending the measurement of faint lines ; 

 moreover, the eye becomes fatigued when regarding for any long 

 period rays of one colour only, and in endeavouring to catch sight 

 of the cross wires on a darkened field. During a spectroscopic 

 examination of the rare earths contained in the mineral rhahdophane 

 ("Chemical Society's Journal" — Transactions, vol. xli., p. 210), a 

 somewhat painful experience led me to devise a means of not only 

 giving relief to the eye, but of greatly facilitating the measure- 

 ment of the lines. I work in a darkened room, the goniometer 

 of the spectroscope being illumined by a shaded lamp which 

 stands to right of the telescope. The grating is movable, while 

 the collimator and telescope are fixed in such a position as to 

 include as small an angle between them as possible. The telescope 

 being to the right of the collimator, a small gas jet is placed upon 

 the left, the rays from which proceed to the grating and are re- 

 flected into the field of the telescope. By the adjustment of this 

 light the field may be illuminated in any colour of the spectrum, 

 and by selecting that tint which is complementary to the colour of 

 the lines to be measured, they are seen to stand out apparently 

 in relief on a bright ground. The intensity of the illumination 

 of the field must, of course, be regulated according to the strength 

 of the lines to be measured. Many of the lines in the spectrum 

 of yttrium obtained from rhahdophane could not have been measured 

 had it not been for this method of working. 



