42 

The work of the Pére Secchi seems designed not so 
_ much to supply this particular want as to give a general 
popular veswmé, of what is known of the physical constitu- 
tion of the Sun. It would therefore scarcely be just to 
measure it by the standard in question, and all the less 
just becausethe very branch of research in which the author 
is most eminent is that of which he speaks the least. 
Only one chapter and part of another are given to spec- 
trum analysis of the solar light and its results, and as the 
operation of spectrum analysis itself is described at some 
length, there is little space left for the discussion of the re- 
sults. Roughly speaking, one half the book is devoted 
to a description of purely optical phenomena as observed 
with a telescope. The appearances and movements of 
the solar spots are in particular treated at great length. 
The other half is devoted to radiation, temperature, gra- 
vitation, spectrum analysis, and the relation of the sun to 
the stars. The chapter on Radiation seems designed to 
save the reader the trouble of referring to elementary 
works on natural philosophy or chemistry, and the next 
has nearly the same object with respect to astronomy. 
Thechapter on Temperature is that which has most piqued 
our curiosity. Theauthor calculates that the temperature 
of the sun must beat least ten millions degrees centigrade. 
On examining the process by which he reaches this con- 
clusion, we find that he sets aside the law of radiation of 
Dulong and Petit, and substitutes that of radiation pro- 
portional to the simple temperature. He gives no reason 
for the adoption of this new law, and we were not aware 
that the other had been disproved. The question is of 
importance, for, if the law of Dulong and Petit were true, 
a sun at a temperature of ten million degrees would 
speedily reduce our earth to vapour. It would be in- 
teresting to measure the temperature of a furnace by the 
effect of its radiation upon a thermometer, in the same 
way that Pére Secchi has measured that of the sun. 
The paper and typography of the work are excellent, 
and among the illustrations are six finely-executed charts 
of stellar spectra, illustrating the author's classification of 
such spectra. 
Mr. Proctor’s work, while covering nearly the same 
general ground with the former, is much more complete in 
its account of recent observations and theories, especially 
of the phenomena of solar eclipses. It is, on the whole, 
better than might have been supposed from its stilted title. 
The first chapter is designed to give an historical dis- 
cussion of the solar parallax. A history of this subject 
at the same time popular, accurate, and complete,is indeed 
much to be desired; but Mr. Proctor’s is imperfect and 
inaccurate in a remarkable degree. He begins very well, 
but grows worse and worse as he approaches his conclu- 
sion. He suddenly stops his history with the year 1868, and 
ignores all that has been said or done since. Confining 
ourselves to two or three paragraphs and a note near the 
close of the chapter, we find the “ligaments,” “black 
drops,” and distortions sometimes seen in interior con- 
tacts of the limbs of Mercury or Venus with that of the 
Sun, described as if they were regular phenomena of a 
transit, without a mention of the facts and experiments 
which indicatethat these phenomena are simple products of 
insufficient optical power and bad definition, which dis- 
appear in a fair atmosphere, with a good telescope well 
adjusted to focus ; and this is followed up with a grave 
NATURE 
[May 18, 1871 

proposal to measure this product of bad definition during 
the next Transit of Venus. One might suppose, from his 
closing statements, that Mr. Stone was the first to “ infer 
from the account given by the different observers, whether 
real or apparent contact was noticed,” and to allow for the 
difference between the two. The subsequent examina- 
tions of the observations used by Mr. Stone are, with a 
single insignificant exception, entirely ignored. We can- 
not, therefore, but wish that the author, before printing 
this chapter, had submitted it for revisal to some one 
acquainted with the subject. 
In the second chapter we find the author more at home, 
We rarely see the accuracy of the mathematician united 
with that vigour and clearness of style so desirable in the 
popular presentation of truth. Mr. Proctor, however, 
here seems to unite both qualifications in a high degree. 
The third chapter gives a very clear and satisfactory 
account of the first principles of spectrum analysis. The 
historical and the logical development of this subject co- 
incide remarkably with each other, and it is therefore that 
very properly adopted in its presentation. We find one 
statement which we must ask leave to doubt, until a more 
satisfactory proof is given than we have yet seen, It is 
that the intensity of the D light (if we may use the ex- 
pression) of incandescent sodium vapour is not only ap- 
parently but actually diminished by passing sun-light 
throughit. If this were so, it would follow that the sodium 
flame not only absorbed the light in question, but that, in 
doing so, it lost the power of emitting it. This would, 
indeed, be a remarkable result. We understand Kirchhoff, 
in the experiment alluded to, to speak only of relative light 
and darkness, and to assert that the D part of the com- 
bined spectrum is less bright than the surrounding and 
intermediate parts. But we cannot conclude from this 
that there was really less light there than when the sodium 
flame shone by itself, as Mr. Proctor does. 
The succeeding chapters give a very full, classified 
summary of recent observations upon the sun, the pro- 
tuberances, the corona, and the zodiacal light. The ac- 
counts of the phenomena observed during total eclipses 
are carried up to that of August 1869. From the pre- 
face it would seem that the work was passing through the 
press in December 1870, and it is a pity it could not have 
been completed by adding the observations of the eclipse 
during that month. The discussion of theories of the 
corona and protuberances is evidently honest, and perhaps 
intended to be complete. He tries to disprove the “at- 
mospheric glare theory” by showing that no part of our 
atmosphere in the direction of the corona is illuminated 
by direct sunlight, a proposition which we apprehend no 
one ever maintained. But we know that every bright 
celestial object is surrounded by a certain amount of stray 
light, due to atmospheric reflection, which increases ra- 
pidly in intensity as we approach the object ; and sucha 
light must therefore surround the real corona and pro- 
tuberances. We also know that every bright object of 
this kind appears larger than it really is, and of a different 
form, from mere optical illusion. Until these two effects 
are eliminated, we can gain no positive knowledge either 
of the exact form or the exact extent of the real objective 
corona. The ‘meteoric theory” of the corona and zo- 
diacal light, sustained by the author, is subject to objec- 
tions as grave as those he brings against other theories ; 
