1872.] Notices of Books. 251 
induction spark,* or the discovery, by this potent means of 
analysis, of elements which had previously escaped recognition. 
The third part of the work occupies two-thirds of the volume. 
The sections treating of the solar spectrum and its lines are well 
written, and being illustrated by reduced copies of Kirchhoff’s 
maps and scale, and of Angstrém’s and Thalén’s maps with 
wave-length scales, are of great value to the student of practical 
spectroscopy. A sufficient account is also given of the atmo- 
spheric lines which make their appearance in the solar spectrum 
when the sun is near the horizon. 
Schellen passes thence to the consideration of the solar spots, 
and, having remarked that it would lead him too far from his 
subject to dwell on their phenomena, naturally proceeds to supply 
about a score of pages of descriptive matter, for which the reader 
might very well have been referred to astronomical treatises. 
The discussion of the spectra of spots and faculz, which follows, 
is full and satisfactory, save that Schellen places rather more 
reliance on Secchi’s observations, and especially on his asserted 
recognition of the presence of aqueous vapour in the solar spots, 
than we believe to be justified by the evidence. Secchi, on one 
occasion, could recognise the bands of aqueous vapour, due to the 
passage of a cloud over the sun, more clearly in the vicinity of 
spots than elsewhere on the solar disc, and therefore somewhat 
hastily concluded ‘‘ that the absorptive power in the sun producing 
these bands is intensified by the absorptive action of the aqueous 
vapour contained in the earth’s atmosphere; and that the cause 
of the absorption in the sun in the neighbourhood of the solar 
spots is therefore the same as that which is present in a fog, 
namely, aqueous vapour.” It does not seem to have occurred to 
him that the general absorption in the spot spectrum would ren- 
der the atmospheric bands apparently more distinct in that spec- 
trum, without any increase in the real darkness of those particular 
bands. The editor remarks, very justly, that Secchi’s result 
appears to him to need confirmation; and we could even have 
wished that he had more pointedly indicated the fallacy in 
Secchi’s reasoning. 
The phenomena of solar eclipses are treated very fully, and in 
a very interesting manner. The sections relating to this part of 
the subject are also abundantly illustrated. We speak of the 
phenomena of eclipses as including the prominences and sierra, 
though in reality the most valuable observations of the last-named 
phenomena have been made when the sun has not been eclipsed. 
In dealing with the history of the method by which this has been 
accomplished, Dr. Schellen falls into some mistakes which are 
corrected by the editor. In particular, it is to be noticed that 
Schellen fails to ascribe the open slit method, by which the actual 
* In this way the 100,000,oooth of a milligramme of strontium, or the 
80,000,000th of a grain of Crookes’s new element, thallium, can be made to 
reveal its presence. 
