1866. | De la Rue and Celestial Photography. 367 
identical in both, thus proving that no change takes place in the 
form of the prominences for a period much longer than the duration 
of a total eclipse. It must be borne in mind that the eclipse 
occurred at an interval of seven minntes between Rivabellosa, where 
Mr. De la Rue was stationed, and Desierto de las Palmas, where 
Padre Secchi was located.* Now, why is it that the photographic 
tablet is impressed by those attenuated images during an eclipse— 
and even by such of them as do not give light enough to the visible 
at the period of totality—and that they do not effect the required 
chemical change upon our sensitive plates when the sun is unob- 
secured ? The only reply which we are at present in a position to 
give is, that the diffused light when the sun is shining is sufficiently 
powerful to overcome the weaker chemical radiations of those solar 
clouds or flames. If this reply approaches correctness, we have 
additional evidence confirming the view that the two principles 
existing in the sun-beam, /ight or luminous power, and actinism 
or chemical power, are not modifications of the same energy—to 
use the accepted term of the day—but rather forces balanced against 
each other, acting indeed in antagonism. That the luminous 
rays have the power of entirely subduing the chemical rays has 
been shown by several experiments by Sir John Herschel + and 
others. 
Referring to the ‘ Report on Celestial Photography in England, 
1859, we find full confirmation of these views. “ Portions of the 
moon, equally bright optically, are by no means equally bright 
chemically ; hence the ight and shade in a photograph do not cor- 
respond in all cases with the light and shade in the optical picture. 
Photography thus frequently renders details visible which escape 
observation optically, and it therefore holds out a promise of a 
fertile future in selenological researches; for instance, strata of 
different composition evidently reflect the chemical rays to a greater 
or less extent, according to their nature, and may be thus distin- 
guished. The lunar surface very near the dark limb is copied pho- 
tographically with great difficulty, and it sometimes requires an 
exposure five or six times as long to bring out completely those 
portions illuminated by a very oblique ray, as others apparently not 
brighter but more favourably illuminated. The high ground in the 
southern hemisphere of the moon is more easily copied than the low 
ground, usually called seas, which abound im the northern hemi- 
sphere. From these circumstances, I ventured in another place 
(‘ Monthly Notices of Astronomical Society’) to suggest, that the 
* « Comparison of Mr. De la Rue’s and Padre Secchi's Eclipse Photographs.” 
By Warren De la Rue, F.R.S.— Proceedings of the Royal Society,’ vol. xiii, 
p. 442. 
+ “Memoir on the Chemical Action of the Rays of the Solar Spectrum,”— 
‘ Philosophical Transactions, 1840.’ 
