ols. PA Se 5 oS age pe oe eee 
H. Draper—Diffraction Spectrum Photography. 405 
scribe fully the method eventually employed in fitting a scale 
to the photograph. 
The wave-lengths of the ultra-violet rays have never, as far as 
I know, been either determined or published except by . 
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: 
3819°0 
3728°8 
3580°2 
3440°1 
3360°2 
3285°6 
. ‘ 3177°5 
These numbers do not entirely coincide in all cases with my 
photograph, as I will show further on. : 
1e detailed results of M. Cornu have not appeared in any 
publication that has reached me. 
DHOVOAEH 
ty that we have thus the means of ascertaining the wave- 
engths of three points, one at each end, and one in the middle 
“ages of the steel points superposed on the spectrum. ; 
Point which had been coincident with D, of the 2d order was 
