Preliminary Table of Wave-lengths of Enhanced Lines. 453 



graphic investigations of metals at high temperatures, dealing specially 

 with the spectra of iron, calcium, and magnesium, and more recently 

 still, * I referred to the enhanced lines of other substances, but re- 

 frained from giving a list of the wave-lengths of the lines photographed, 

 as the series of comparisons with the large Rowland grating was not 

 then completed. 



The important part which the enhanced lines of the elements play 

 in the study of stellar spectra cannot be over-estimated, but a great 

 advance can only be made in this direction by a systematic examina- 

 tion of the spectra of all the elements. Such an undertaking as this 

 involves considerable time and labour. I have been fortunate enough 

 to have the use of the large 42-inch Spottiswoode coil for a short space 

 of time, and employed it in this work, for which it is specially adapted, 

 as the brilliancy of its spark shortens the time of exposure. Although 

 I have previously stated my indebtedness to Mr. Hugh Spottiswoode 

 and Mr. Gr. Matthey for their assistance, I wish again to express my 

 best thanks to them, and I must now add Professor Moissan and Sir 

 William Crookes, who have kindly supplied me with some specimens 

 of metals. 



The elements which have been dealt with in this investigation are 

 the following : — " Aluminium, bismuth, chromium, copper, iron, mag- 

 nesium, manganese, titanium, and vanadium."' 



For each of these elements the spark and arc spectra were photo- 

 graphed and compared, and the wave-lengths of the enhanced lines, 

 that is, those lines which are intensified in passing from the temperature 

 of the electric arc to that of the spark, were determined. 



Method of Reduction. 



The method of reduction was as follows : — The spark spectrum of 

 the element was first compared directly with the spark spectrum of air 

 between platinum poles, and the air lines were thus eliminated. The 

 spark and arc spectra of the element being taken on different plates, 

 were then compared, and the lines present in the spark spectrum but 

 absent from the arc, or lines relatively more intense in the spark than 

 in the arc spectrum, were noted. The wave-lengths of these lines were 

 then determined by direct comparison with a solar spectrum taken 

 under the same instrumental conditions, and reference to Rowland's 

 list of lines in the normal solar spectrum. 



In siru merits Emplo yed. 



The grating used is a 6-inch concave one, having a surface 2 inches 

 by 5 J inches, ruled with 14,438 lines to the inch, and a radius of 

 curvature of 21 feet 8 \ inches. The instrument is mounted in the 

 * ' Roy. Soc. Proc.,' vol. 61, p. 441. 



