294 PROCEEDINGS OP THE AMERICAN ACADEMY 



light from the spark. From this negative the value of the wave- 

 lengths of the faint lines were obtained by interpolation from the 

 values of the stronger lines as determined by the first plate. It also 

 served as the final test whether the lines on the first negative were 

 of the first or second order. All of the lines more refrangible than 

 line 2123.1 were in the case of copper found from this negative and 

 from line 2136.1 by direct measurement. 



Another method of distinguishing which lines on the first negative 

 belong to the second and which to the first spectrum, is to place in 

 front of the slit while taking the metal lines a piece of plane glass. 

 The second spectrum for this refrangibility will be completely cut out, 

 and only the metal lines of the first remain, being in the visible violet. 



The only source of error was in the setting of the microscope upon 

 the broad or faint lines. The probable error of this is about .1 tenth 

 meter. For the few most refrangible lines it may be greater. 



Effect of Change of Temperature of Source of Light on Constancy of 



Position of Metallic Lines. 

 In the process of the investigation we were much troubled by a 

 slight shifting in position of the metallic lines upon the photographs. 

 This shifting could be observed when the metallic lines were com- 

 pared with a solar spectrum taken upon the same plate. The amount 

 of this shifting in no case amounted to more than .1 or .2 of a wave- 

 length. At first we thought it might be possible that there was a 

 change in refrangibility of the metallic lines due to a difference in 

 temperature of the source of light, and a long study was made of 

 the influence of the temperature of the source of light upon its wave- 

 length. When a metal was burned in the carbon electric light with 

 varying strength of current, no displacement could be observed between 

 the lines of the metal photographed beneath each other upon the same 

 sensitive plate. When the electric spark with a large battery of Ley- 

 den jars was substituted for the electric arc, and the metallic lines 

 obtained by the light of the spark were compared with those from the 

 arc, occasionally a small displacement could be observed. This did 

 not seem to arise from a change of position of the source of light, or 

 from the heating of the slit of the spectroscope. A careful study of 

 the iron lines showed us that the wave-length of the iron lines in the 

 sun and those obtained from burning iron in the electric arc were the 

 same to certainly one hundredth of a wave-length. The displacement 

 we observed was noticed only when the electric spark was employed! 

 This shifting did not arise from a change of position of the spark in 



