Dr. H. Draper on Diffraction- Spectrum Photography. 421 



stretched unequally in printing; and on applying a photographic 

 reduction to my spectra, coincidence could not be obtained. As, 

 however, the subject of providing a scale for these diffraction- 

 spectra is of prime importance in giving value and precision to 

 the wave-lengths presented in this memoir, I propose to describe 

 fully the method eventually employed in fitting a scale to the 

 photograph. 



The wave-lengths of the ultra-violet rays have never, so far as 

 I know, been either determined or published except by J. W. 

 Draper in 1844, Mascart in 1866, and Cornu in 1872. J. W. 

 Draper's memoir has a steel engraving of some of the principal 

 lines, from which the wave-lengths may be approximately read. 



The large plate which accompanies Mascart's long and valu- 

 able memoir is of the prismatic spectrum ; but he furnishes in 

 addition the following Table of wave-lengths: — 



L 3819-0 



M 3728*8 



N 3580-2 



O 34401 



P 3360-2 



Q 3285-6 



R 3177-5 



The numbers do not entirely coincide in all cases with my 

 photograph, as I will show further on. 



The detailed results of M. Cornu have not appeared in any 

 publication that has reached me. 



I have used as a basis the numbers given by Angstrom for 

 the rays D 2 , b 4 , and G; and if there should be any small error 

 in his determinations, my scale will require a proportionate cor- 

 rection, which can easily be effected. At first it seemed better 

 to take .G and H as fixed points ; but the line H is so broad, 

 and has so many component lines, that its position is uncertain, 



o 



and, moreover, being almost at the limit of visibility in Ang- 

 strom's apparatus, it was more open to errors of measurement. 

 These reasons led me to take advantage of the fact that the 

 second spectrum overlaps the third, the ray D of the second 

 being near H of the third, and b of the second near of the third. 

 It is obvious that we have thus the means of ascertaining the wave- 

 lengths of three parts — one at each end, and one in the middle of 

 my photograph. As the rays D and b cannot impress them- 

 selves on collodion by any length of exposure that it is conve- 

 venient to give, and as in my method of working the ultra-violet 

 rays could not be seen simultaneously with them, it was neces- 

 sary to resort to the following device. I placed in front of the 

 sensitive plate and close to it two fine steel points, one of which 



