AND ITS RELATION TO SOLAR SPECTRA. 49 



10 and 9 respectively in the scandium arc, while in the euxenite arc the former was 

 less than half the intensity of the latter. Such differences as this are no doubt to be 

 accounted for by differences in the conditions of experiment. LOCKYER and 

 BAXANDALL worked with scandium oxalate, and probably obtained what may be 

 properly considered the arc spectrum of the element. In euxenite, on the other hand, 

 the presence of calcium and other substances tended to produce the condition of a 

 " naming arc," and the scandium lines doubtless appeared with intensities approxi- 

 mating to those proper to the "arc-flame" or "flame" spectrum. That is, in 

 euxenite the presumably "enhanced lines"" were notably weakened as compared 

 with their intensities in the true arc spectrum. Further evidence that these 

 weakened lines were enhanced lines was given in some cases by comparison with the 

 spark intensities of THALEN. 



Although a comparison of intensities estimated by different observers is often apt 

 to be misleading, the " inversions " in the case of several scandium lines were 

 sufficiently pronounced to support the previous deduction, from the behaviour of other 

 elements, that flame lines were strengthened and enhanced lines weakened in spots, 

 while the enhanced lines might appear alone in the upper chromosphere. 



At this stage of the investigation I was fortunate enough to receive the valuable 

 assistance of Sir WILLIAM CKOOKES. In reply to an inquiry about two years ago 

 Sir WILLIAM CROOKES informed me that scandium was extremely scarce, and that, 

 although he had been carefully collecting residues for some time, he had not then 

 obtained sufficient for a satisfactory examination of the spectrum. By the end of last 

 year, however, when I acquainted him with the results obtained from euxenite, lie not 

 only informed me that he had succeeded in collecting and purifying a considerable 

 amount of scandia (Sc 2 O 3 ), but very generously placed half a gramme of the substance 

 at my disposal. It thus became possible to make a much more complete catalogue of 

 scandium lines, and a more satisfactory investigation of the behaviour of the lines 

 under varying experimental conditions. As Sir WILLIAM CROOKES was himself 

 occupied with the spark spectrum, the electric arc was exclusively employed in my 

 own work, but the conditions were varied so as to give in some cases an approach to 

 the spark spectrum, and in others to the flame spectrum. 



The Spectrograph Employed. 



The photographs were taken with a spectrograph of Littrow form, having a lens of 

 12 feet focal length, and one prism of GO for which /A D is l'G4G7. The plates are 

 12 inches in length, and show satisfactory definition throughout, the region covered 

 being 3930 to 4G70, or 4G70 to GGOO, according to the position of the mirror which 



* " Enhanced lines " are lines which are relatively strengthened in passing from the arc to the spark 

 (LOCKYER). 



VOL. CCIX. A. H 



