460 Scientific Intelligence. 
Fizeau gave the number 0°001445 millim. for the wave-length of 
a band which appears to be the band A’. On the other hand, 
my father obtained for this same band, by the phosphorographic 
method, numbers varying between 0°001400 and 0:001200 millim., 
and gave the number 0°001220 millim. for its most refrangible 
border. It was therefore necessary to determine these wave- 
lengths directly by using a grating. 
n 1879 bney prepared a very beautiful map of the 
infra-red portion of the normal spectrum; it was obtained by 
photography, and extended to about the wave-length 0°000980 
illim. In my previous researches I was enabled, by the appli- 
cation of phosphorescence, to measure, in the diffraction-spectrum - 
of a grating, to about 0°001000 millim.; but the feeble brilliancy 
of the spectra did not permit of my going further, and I adopted 
the number 0°001220 millim. for the most-refrangible border of 
the band A” which can be easily observed in the spectrum formed 
by a prism. The numbers above 0°001000 millim., published in 
the paper cited above, were obtained by means of an interpolation 
based upon this assumption. 
In a memoir published in 1883 Mr. Langley deduces the wave- 
lengths of the bands A’, A”, A’, Aiv i ion 1 
ose W 
disposal during my former researches. These substances have 
allowed of my measuring, with a quite near approximation, 
e t 
finer lines; the numbers given by these experiments are near 
reater. 
The solar rays, concentrated upon a narrow slit, in the focus of 
a collimator were made to fall upon a beautiful grating, ruled on 
metal by Mr. Rutherfurd, which M. Mascart was kind enough to 
lend me. The rays, brought to convergence by a lens, then 
transversed a bisulphide-of-carbon prism, of which the edges were 
at right angles to those of the slit and the lines of the grating, 
and formed upon the phosphorescent substance a series of oblique 
spectra, in which the rays of the spectra of different orders were 
in juxtaposition and not superposed. The slit was sufficiently 
narrow to allow of distinetly seeing the principal lines of the 
luminous spectrum; and on comparing the position of the lines 
and bands in the infra-red part of the first spectrum with those 
of the known lines in the luminous part of the spectra of second 
and third order, their wave-length was obtained with an approxt- 
mation which depended only on the accuracy of the scale-reading. 
