178 J. W. Praper—Phosphorograph of a Solar Spectrum. 
impress of a solar spectrum they present patches of light and 
shade irregularly distributed. Though in a general way they 
confirm the statements made above, they do not do it sharply 
or satisfactorily. 
the yellow,—I found that this rectangle is not given by 1 
and 2. In 8 it is doubtful. In 4 it is quite visible, and in 5 
and 6 strikingly so. 
Is the blackening then due to heat? That it occurs beyond 
the violet, that is, beyond the lines H, seems to render such an 
opinion doubtful, for it is commonly thought that the effect of 
heat is not recognizable there. And in the phosphorographic 
spectroscope I have used, the optical train, prism, lenses, etc., 18 
of glass, which must of course exercise a special selective heat- 
absorption ; but the traces of this in the phosphorograph Tcould 
never detect. 
In the diffraction spectrum, I had attempted nearly forty 
years ago to ascertain the distribution of heat (Phil. Mag., 
March, 1857), but could not succeed with the experiment in a 
completely satisfactory manner, so small is the effect. I 
ec 
Now, considering the exceedingly small amount of heat 
available in this case, and considering the intensity of the effect, 
is there not herein an indication that we must attribute this 
result to some other than a calorific cause ? 
T endeavored to obtain better information on this point by 
using the rays of the moon, which, as is well known, are very 
deficient in heating power. Many years ago I had obtained 
some phosphorographs of that object. With the more sensitive 
preparations now accessible, and with a telescope 11 inches 1? 
aperture and 150 inches focus, there was no difficulty in pro 
curing specimens about 1:4 inch in diameter. These repre 
sented the lunar surface satisfactorily. At half-moon an expos- 
ure of three or four seconds was sufficient to give a fair prool. 
But, on insolating a phosphorescent tablet, and causing abe 
converging moon rays to pass through the red glass which I 
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