50 ME. A. FOWLEE ON THE SPECTEUM OF SCANDIUM, 



returns the light through the prism. At 6600 the linear dispersion is 127 tenth- 

 metres to the millimetre, while at 3930 it increases to 1'8 tenth-metres per millimetre. 



The determination of wave-lengths was made in the usual manner with the aid of 

 the Cornu-Hartmann interpolation formula for prismatic spectra, a separate equation 

 being computed for each region of 100 to 200 tenth-metres. Lines in a comparison 

 spectrum of iron, slightly overlapping that of scandium, were used as standards, 

 ROWLAND'S solar wave-lengths for corresponding lines being adopted. Numerous 

 iron lines were included in the measures, and small corrections, depending upon the 

 degree of agreement between the computed and observed values of these extra lines, 

 were applied to the resulting wave-lengths for scandium. 



On comparison with LOCKYER and BAXANDALL'S wave-lengths, it was found that 

 there were small systematic differences in some parts of the spectrum which persisted 

 after repeated measurements of different plates. It was accordingly thought desirable 

 to take another photograph in which iron was mixed with scandia, so that there 

 should be no possibility of a relative displacement of the comparison lines. The 

 previously deduced wave-lengths were fully confirmed by this procedure. 



For all but the weakest lines it is hoped that in the part of the spectrum more 

 refrangible than D the wave-lengths are correct to within two or three hundredths of 

 a tenth-metre, while on the red side of D, in consequence of the smaller dispersion, 

 the errors may be somewhat greater. 



The Arc Spectrum. 



The arc .spectrum was produced in the ordinary manner by introducing a small 

 quantity of scandia between carbon poles, the current being obtained from the 

 110-volt lighting circuit. The P.D. between the poles was about 40 volts, and the 

 current 8 amperes. The scandia did not volatilise very readily, and with the small 

 amount of material that one felt justified in using, carbon flutings were also present 

 in the spectrum and tended to conceal some of the fainter metallic lines. Better 

 results were obtained by mixing a little silicate of soda or silicate of potash with the 

 scandia, the carbon flutings being then practically eliminated, and a good spectrum 

 secured with a very small amount of scandia. The few impurity lines introduced in 

 this way were readily recognised, and were less objectionable than the multitude of 

 lines composing the carbon flutings. To avoid unnecessary waste of so rare a 

 substance, the arc was usually enclosed in a glass globe, so that the residues might be 

 collected and the scandia subsequently separated by chemical treatment. 



A list of the arc lines between K and C is given in Table I., at the end of this 

 paper. The intensities are on a scale such that 10 corresponds to the brightest lines, 

 and 1 to lines which are just well visible on negatives taken with moderate exposures ; 

 very faint lines, which only appeared clearly on strong photographs, are indicated, on 

 ROWLAND'S plan, by and 00. The estimates of intensity are based on an exami- 



