New Table of Standard Wave-lengths. 51 



and A(wv) denote lines due to absorption by the oxygen and 

 the water-vapour, respectively, in the earth's atmosphere. 

 An ? mark after an element means that it is doubtful if the 

 line is really due to the element named. If two elements are 

 given on the same line (e.g. Mn-Di w.l. 3295*957), it is to 

 be understood that they have apparently coinciding lines at 

 that particular wave-length. If two or more elements are 

 bracketed 



Mn) 

 e.g., Ti Vw.l. 5260*384, 



Fe ) 



it means that the first one has a line coinciding with one side 

 of the corresponding line in the solar spectrum, the second 

 one has a line coinciding with the middle, &c, and the 

 appearance of the solar line itself is given in a later column. 

 An ? standing alone denotes that the element which corre- 

 sponds to the given wave-length is unknown. 



The second column gives the intensity of the line in the 

 arc spectrum ; the third its appearance, and the fourth and 

 fifth do the samefor the line in the solar spectrum. R stands 

 for " reversed ; " N, nebulous ; ,d, double ; t, triple ; ?, doubt- 

 ful or difficult. The size of the number indicates to some 

 extent the intensity of the line. For instance the intensity 

 10 means that the line is apparently 10 times as intense as 

 the intensity 1. Measurements- of intensity by eye-observa- 

 tions, direct or on photographic plates, are of course most 

 uncertain. And so the figures given are estimates which do 

 not apply to comparisons of different portions of the spec- 

 trum, but are intended to give some idea of the relative 

 effects. The intensity of some lines in the arc spectrum of a 

 given substance, e. g. Ca, is often so much greater than that 

 of the others, that the absence of some lines in the solar 

 spectrum is easily understood. The sixth column gives the 

 character of the standard. M means that the line is a 

 standard in the arc spectrum ; means that the line is an 

 ordinary solar standard ; ©', a better solar standard ; © r/ , a 

 remarkably good solar standard ; and 0i a rather poor 

 solar standard. 



The next two columns give the " weights " to be attached 

 to the values of the wavelengths as standards in the arc and 

 solar spectra, respectively. 



The last two columns give the final values of the wave- 

 lengths measured in Angstrom units, i. e., in ten millionths 

 of a millimetre in ordinary air at about 20° C. and 760 millini. 

 pressure. 



Notes marked J. are by Mr. Jewell. 



E2 



