7 > 5 is a Gis a pint - rey rote ’ ian “t ti ies 
fei NATURE | ¥une 5, 1873 
well explored localities, we probably have even now a 
very fair representation of the littoral Echini of the world. 
It would of course be rash to make any predictions as to 
the number of new forms that will doubtless be brought to 
light by the researches of Wyville Thomson—but these 
will probably be deep-sea forms. Did space allow we 
would gladly have dwelt longer on this most interesting 
portion of Agassiz’s memoir. 
The total number of genera adopted is 90, with 207 
species. The atlas accompanying these parts contains 
49 plates—the first seven are devoted to charts, repre- 
senting the distribution of the Echini throughout the old 
and new worlds, and the remaining portion to figures of 
some of the new or little known species. Some of the 
plates are photographs—and very excellent ones — others 
are photo-printed by the albert type process, and while 
these have scarcely the brilliancy or evenness of detail 
as such engravings as those of Echini in the expedition to 
Egypt, yet when the enormous difference in cost is taken 
into account, these photo-printed plates must be a subject 
of congratulation to the working and not over-rich natura- 
list. Some others of the plates are lithographed from 
Agassiz’s drawings, and these we would select as being the 
most useful in this atlas. 
Next we would mention a very important paper by 
Prof. Lovén, published in “ Ofversigt af Kongl. Veten- 
skaps-Akademiens Férhandlingar,” 1871, No. 8. This 
paper was read on June 14, 1871, but was not, we think, 
published until the summer of 1872, and as a translation 
of it in full by Mr. Dallas has been published in the 
“ Annals and Magazine of Natural History,” vol. x., 4th 
series, October to December 1872, we will but very 
briefly allude to it here. Prof. Lovén describes some 
very small spheroidal button-like bodies furnished with a 
short stalk, which is normally attached to a small, slightly 
projecting tubercle, which he calls Spheridia; these occur 
apparently in all Echinoidea except Cidaris ; they are 
fully described as they occur in the different families. 
Lovén next describes the order which prevails in the dis- 
position of the ambulacral plates throughout the whole 
class, for which he even gives a formula. 
Passing from the sea urchins to the Brittle stars, we 
have also, from the Proceedings of the Royal Academy 
of Stockholm, a paper by Ljungman describing the col- 
lection of Ophiuroids made by Dr. Goés in the West 
Indies, in the Josephine Expedition, Fifty-seven species 
are enumerated, of which fifteen are described as new. 
Many of these latter were dredged from very considerable 
depths. The author adds to his paper a conspectus of 
the genera of Ophiodermatidz and a conspectus of the 
Atlantic species of the genera Amphiura and Amphi- 
pholis. 
Liitken, in an important memoir published in the Pro- 
ceedings of the Royal Academy of Copenhagen, Part 2, 
1872, entitled “ Ophiuridarum novarum vel minus cogni- 
tarum descriptiones nonnullz,” describes a number of 
new species from different parts of the world, as well as 
gives some details of little known species. To this 
memoir there is appended a chapter “ On Spontaneous 
Division in the Star Fishes,” at the conclusion of which 
the author sums up with the following general proposi- 
tions :—(1), The most energetic manifestations of the 
faculty of regeneration in animals is the power of divisi- 
bility ; (2), In certain forms of Radiates, in which the 
faculty of regeneration is very highly developed, spon- 
taneous division takes place alone, as in Ophiuroids and 
Asteroids, or together with gemmation, as in Actinia ; 
(3), Actual spontaneous division or “ Schizogony,” in the 
Actinia, Medusa, Asteroids, and Ophiuroids (which must 
not be confounded with the disguised forms of gemmation 
met with in Infusoria and certain Chetopods) may be 
regarded as a peculiar form of Agamic reproduction such 
as Blastogony, Sporogony, and Parthenogony. 
Lastly we have to mention the appearance of a modest 
catalogue of Echinodermata of New Zealand, with 
diagnosis of the species, by Capt. F. W. Hutton, F.G.S., 
Assistant Geologist, Colonial Department. In it thirty- 
four species are described, eighteen of them being de- 
scribed as probably new to science. 
E, PERCEVAL WRIGHT 
ON THE SPECTROSCOPE AND ITS 
APPLICATIONS 
xX. 
I HAVE not yet done with the spot-spectrum re- 
ferred to in last article, Not only is there gene- 
ral absorption, but there are indications of increased 
selective absorption in the case of the line D, as 
I could also show if I were dealing with the iron 
lines, the magnesium lines, or the other well-known 
lines of the solar spectrum. Not only, then, have we a 
general absorption, increasing as the middle of the sun- 
spot is approached, but this sodium line D is also thick- 
ened, so that we have, as a result of a single examination 
of a single sunspot, the fact that a sunspot is due to 
general absorption, #/us special absorption in some par- 
ticular lines. 
Now, in what I said some time since on the radiation of 
hydrozen, I pointed out to you that the F line of hydrogen 
was different from the C line—in fact, I showed that it 
widened out towards the sun—and I also told you that 
Dr. Frankland and myself have asserted that that widen- 
ing out is due to pressure, and we have been able artifi- 
cially to widen out this F line of hydrogen by increasing 
the pressure. Now it struck us that possibly we might 
find some connection between that widening out of the F 
line of hydrogen and the widening out of the sodium line 
in the spot which I have just shown you. There is an 
experiment by which it is perfectly easy for us to repro- 
duce this artificially, so that you see we can begin at the 
very outside of the sun by means of hydrogen, and see 
the widening of the hydrogen lines as the sun is ap- 
proached ; and then we can take the very sun itself to 
pieces, and, by examining the pieces, see that the sodium 
lines vary in thickness in different parts of the spot, as 
the hydrogen does outside the spot region altogether ; in 
fact, the pressure is continually increasing down in the 
spot exactly in the same way as it increases in the hydro- 
gen envelope towards the sun. ; 
If we take a tube containing some metallic sodium 
sealed up in hydrogen, and pass a beam of light from the 
electric lamp through it, by decomposing this beam with 
our prisms we shall obtain an ordinary continuous spec- 
trum without either bright or dark lines, but by heating 
the metallic sodium in the tube which is placed in front 
of the slit, we really fill that tube with the vapour of 
sodium ; and as the heating will be slow, the sodium 
vapour will rise very gently from the metal at the bottom, 
so that we shall get layers of different densities of sodium 
vapour filling the tube. Immediately the sodium begins 
to rise in vapour, a black absorption line shows itself in 
our spectrum in precisely the same position as the yellow 
line of sodium, and you will find that the thickness of 
the sodium absorption line will vary with the density 
of the stratum of vapour through which the light passes. 
Thus from the upper part of the tube we obtain a fine 
delicate line, which gradually thickens as we approach the 
bottom ; and thus we reproduce-the appearance in the 
spectrum of the spot where the layers of sodium vapour 
are very dense, and the very fine delicate line of the 
sodium vapour when thrown up into the sun’s chromo- 
sphere, 
We must next speak of what happens in the case of the 
magnesium lines. A very obvious magnesium line is 
lettered 4 in the solar spectrum. 
rated by different intervals. There is a very impor- 
It is a triple line, sepa- 
