J. W. Draper—Phosphorograph of a Solar Spectrum. 181 
the bichromate in long exposures permits a sufficiency of the 
more refrangible rays to pass, to produce a marked photo- 
graphic effect; and hence I feared that any experiments sup- 
posed to prove the existence of lines in the infra-red would be 
open to the criticism that they, in reality, belonged to the more 
refrangible regions of the spectrum of the second order, and 
that a satisfactory examination of the case would exclude the 
use of the grating and compel that of the prism. With the 
prism I could not obtain clear evidence of the existence of more 
than three lines, or perbaps groups, and doubtful indications 
of a fourth. If in these examinations we go as far as wave 
length 10,750, the limit of Captain Abney’s map, we nearly 
reach the line H’ of the third spectrum. This would include 
all the innumerable lines of spectrum 2, and even many of those 
of spectrum 3. In such a vast multitude of lines, how would 
it be possible to identify those that properly belonged to the 
first, and exclude those of the second and third spectra ? 
Besides, do we not encounter the objection that this is alto- 
gether beyond the theoretical limit of the prismatic spectrum ? 
is brings us to Captain Abney’s recent researches, which, 
by the aid of the grating, carry the investigation referred to the 
prismatic spectrum as far below the red as the red is below the 
yellow. They are not to be regarded as an extension of 
exploration in the infra-red region,—for they really do not 
carry us beyond my own observations in 1842,—but as securing 
the resolution of these lines or bands into their constituent ele- 
ments. J had never regarded them as really single lines. The 
breadth or massiveness of their photographs, too, plainly sug- 
gests that they are composed of many associated ones. The 
principle of decreasing refrangibility with increasing wave 
length incapacitates the prism from separating them, but the 
grating which spreads them out according to their wave length 
reveals at once their composite character. 
In Captain Abney’s map, after leaving the red line A, we 
find three groups: (1) ranging from about 8150 to 8350; (2) 
from 8930 to 9300; (8) from 9350 to 9800. These, admitting 
that the lines of the subsequent grating spectra have been 
excluded, are then the resolution of a, f, 7. 
_I suppose that care has been taken to make sure of that, 
either by absorbent media or by a subsidiary prism. If the 
grating had been ruled in such a manner as to extinguish the 
spectrum, inconveniences would arise from the char- 
acteristics thereby impressed on the firs ; 
n the phosphorographic spectrum on luminous paint, this 
vast multitude of lines is blended into a mass which probably 
can never be completely resolved into its elements, on account 
of the propagation of phosphorescence from particle to particle, 
T have resolved it into two or three constituent groups, and fre- 
