180 J. W. Draper—Phosphorograph of a Solar Spectrum. 
account of their discovery, not knowing that it had been made 
and published long previously in America 
Sir J. Herschel had made some investigations on the distri- 
bution of heat in the spectrum, using paper blackened on one 
side and moistened with alcohol on the other. He obtained a 
series of spots or patches, commencing above the yellow and 
extending beyond the red. Some writers on this subject have 
considered that these observations imply a discovery of the 
lines a, 8, 7. They forget, however, that Herschel did not use 
a slit, but the image of the Sun,—an image which was more 
than a quarter of an inch in diameter. Under such circum- 
stances, it was impossible that these or any other of the fixed 
lines could be seen. 
I have many times repeated this experiment, but could not 
obtain the same result, and therefore attributed my want 0 
success to unskillfulness. More recently Lord Rayleigh (P hil. 
Mag., November, 1877), having experimented in the same 
direction, seems to be disposed to attribute these images to a mIs- 
leading action of the prism employed. Whatever their cause 
may be, it is clear that they have nothing to do with the fixed 
lines a, 8, 7, now under consideration. 
In these experiments, and also in others made about the same 
time on the distribution of heat in the spectrum, I attempted to 
form a diffraction spectram without the use of any dioptric 
media, endeavoring to get rid of all the disturbances which 
arise through the absorptive action of glass by using as the grat- 
ing a polished surface of steel on which lines had been ruled 
with a diamond, and employing a concave mirror instead of an 
ference of heat, no one as yet has made reference to these 
.” Nearly thirty years before the date of his memoir I had 
published an engraving of them. (Phil. Mag., May, 1843.) 
After I had discovered these three lines, I intended to use the 
grating for the exploration of that region, since it extends 1f, 
ar more than the prism can do; but, on making the attempt, 
was discouraged by the difficulty of getting rid of the more 
refrangible lines belonging to the second spectrum. I had 
hoped to eliminate these by passing the ray on its approach to 
the slit through a solution of the bichromate of potash. But 
