48 Mr. J. S. Ames on some Gaseous Spectra. 



where the coefficients rapidly grow smaller. This formula is> 

 of course, the analytical statement that for lines near the 

 head of a band the second differences of the wave-numbers are 

 constant. So, if these series and bands represent in any 

 degree the fundamental or original vibrations of the " mole- 

 cule," as I think probable, it is of interest to note that a 

 similar law applies to both. The exact law of distribution 

 which must come from theoretical considerations will there- 

 fore include this as a special case. 



Addendum. — Since writing the above the Philosophical 

 Magazine for April (1890) has reached me, and it contains 

 an article by Dr. Rydberg, of the University of Lund, " On 

 the Structure of the Line-Spectra of the Chemical Elements," 

 which demands notice. In this paper, which is only pre- 

 liminary, Dr. Bydberg calls special attention to the many 

 series in the spectra of the elements, divides them into various 

 classes, gives formulas for their distribution, and states that 

 the wave-lengths are periodic functions of the atomic weights. 

 This last relation, which is barely indicated in the article, 

 seems to be nothing but the one of M. de Boisbaudran, which 

 has already been discussed*. The law of distribution is 

 nothing but Banner's slightly generalized. The valuable part 

 of the paper lies in the division of the lines of the spectra into 

 different series and in showing the relations between them. 

 He ' does not state how he gets his wave-lengths ; but they 

 seem to be a combination of Hartley's and Liveing and 

 Dewar's. So he has confused the spark- and arc-spectra. 



Y. On some Gaseous Spectra :— Hydrogen, Nitrogen. By 

 Joseph Sweetman Ames, Assistant in Physics, Johns 

 Hopkins University f . 



MY investigations on the spectra of gases were begun at 

 the suggestion of Professor Rowland, and were designed 

 primarily to ascertain the exact wave-lengths of the line- 

 spectrum of hydrogen. Incidentally some other spectra have 

 also been studied. 



Metliod and Apparatus. 



A concave grating, mounted in the well-known Rowland 

 manner, was used. It was ruled especially for this research, 

 was 6 inches (15*2 centim.) wide with 20,000 lines to the 

 inch, and had a radius of curvature of 13*5 feet (4*11 metres). 



The experiments were conducted according to the method 



* Pp. 33 et seq. 



t Communicated by the Author. 



