
Oct. 5, 1871] 
_ satellite has been on the disc of Jupiter, and by Dawes 
_-and Secchi even in other positions, may be of changeable 
character. Ata mean II. is relatively the most, 1V. the 
least Juminous. As to their micrometrical measurement, 
every one who is acquainted with the telescopic aspect of 
these minute discs will readily comprehend its difficulty. 
It has, however, been attempted in various ways, but not 
by the double-image micrometer, which does not seem 
_ to have been used ; the results, as may be expected, pre- 
sent considerable discrepancies, but the final values ob- 
tained by a combination of different methods in the hands 
_ of various observers are as follow :—I., 1/081 ; II.,0”g10; 
III., 1537; IV., 1282 ; or, in English miles, 2,498, 2,102, 
3,551, 2,962, the solar parallax being taken as 8'"go. 
These values, all things considered, differ so little from those 
given by Lockyer (Guillemin’s ‘ Heavens”)— namely, 
2,440, 2,192, 3,759, 3,062—that we may consider ourselves 
possessed of a very fair approximation to their real mag- 
nitudes. 
Astothe “albedo” of their surfaces, I. shows no great varia- 
tion ; it falls, according to Zéllner’s estimate of the reflective 
power of terrestrial materials, between that of marland white 
sandstone ; II. has the greatest variations of albedo, which 
at a mean somewhat exceeds that of white sandstone ; III., 
the variations of which are smaller andmore regular, comes 
between marl and quartzose porphyry ; IV., which varies 
least, equals that of moist arable land. It will probably 
be thought, however, that curious as these comparisons 
may be, the standards are much too uncertain to give any 
Satisfactory result. As to colour, Dr. Engelmann, after 
citing the elder Herschel’s estimates—I., white ; II., white, 
bluish, and ash-coloured ; III., white ; 1V., dusky, dingy, 
inclining to orange, reddish, and ruddy—specifies as the 
determination of other observers : I., yellowish ; II., white 
or yellowish ; III., intensely yellow with low powers ; IV., 
in achromatics a distinct dusky blue. (These colour- 
values at any rate afford no countenance to the common 
impression that Herschel had a bias for red tints.) To 
the writer, whether with two achromatics, or a nine-inch 
silvered mirror, this satellite has always appeared ruddy 
when its colour has formed the object of notice ; in such 
discrepancies something may be instrumental, something 
subjective. Itis pleasant to see here a very full appreci- 
ation of the laborious perseverance and honest accuracy 
of the labours of Schréter, to whose merit time seems to 
be doing tardy justice ; no notice is taken, however, of the 
observations of Gruithuisen, who twice appears to have 
seen spots on III on the background of the sky ; nor is 
reference made to the irregular shape of that satellite re- 
marked by Secchi and his assistant ; nor to the apparent 
discrepancy which has often been noticed between the 
magnitudes of the satellites and their shadows. Still, the 
treatise may be considered as very nearly an exhaustive 
one ; and a most important and acceptable contribution 
to planetary astronomy. It may be added that it contains 
avery valuable determination of the telescopic magnitude 
of Jupiter, from the average of eleven observers ; the re- 
sult being, with the double-image micrometer 37/609 for 
the equatorial, 35/°236 for the polar diameter; with 
the wire micrometer, 38312 and 35914: the former 
values, which he seems to prefer, exhibiting a flattening 
of T. W. WEBB 
15°82. 
NATURE 

443 
OUR BOOK SHELF 
Transactions of the Geological Society of Glasgow. Vol. 
III. Supplement. On the Carboniferous Fossils of the 
Westof Scotland : their Vertical Range and Distribution. 
By John Young, Vice-President. Witha General Cata- 
logue of the Fossils and their Mode of Occurrence, 
and an Index to the Principal Localities. By James 
Armstrong, Honorary Secretary. (Glasgow, 1871.) 
THIS catalogue of fossils will doubtless be of great use 
not only to local geologists, but to others ata distance, who 
may desire to compare the treasures of English and Irish 
Carboniferous strata with what the equivalent beds in Scot- 
land have yielded. So far as they go, the lists appear to 
be drawn up with considerable care, and Mr. Armstrong 
is to be congratulated upon the result of what must have 
been somewhat laborious work. But we are sure he will 
be the first to admit that much, very much, still remains 
to be done before the Scottish Carboniferous flora and 
fauna can be satisfactorily compared with those of other 
countries. We are constantly being reminded throughout 
this catalogue that not only in private collections, but also 
in public museums in the West of Scotland, there are 
numbers of specimens under almost every class waiting 
to be identified, amongst which there is every reason to 
believe that not a few are species new to science. This, 
it seems, is specially the case with the plants, the rich 
flora of the Carboniferous period being represented sin the 
catalogue by only ninety species. But Mr. Carruthers, 
we are told, hasseveral undescribed specimens in hand, of 
which we shall, no doubt, hear by-and-by. The fishes, 
it would appear, also need looking after. There are 
eighty-four species, under forty genera, named in the 
catalogue ; but a large number in various collections have 
never been correctly identified with described species, and 
Mr. Young expresses a hope, in which we cordially join, 
that Prof. Young will be induced to prepare a special 
catalogue of these and the Reptilia, of which only seven 
species are given by Mr. Armstrong. The other classes 
are represented as follows :—Foraminifera, 2 genera, 4 
species ; Hydrozoa, 1 g. 2 sp.; Zoophyta, 22 g. 59 sp. ; 
Echinodermata, 6 g. 15 sp.; Annelida, 4 g. 7 sp. ; Crus- 
tacea, 19 g. 71 sp.; Insecta, 2g. 2 sp.: Polyzoa, 11 g. 
36 sp.; Brachiopoda, 15 g. 50 sp.; Lamellibranchiata, 
28 g. 127 sp.; Pteropoda, I g. I sp.; Gasteropoda, 
I5 g. 75 sp.; Cephalopoda, 6 g. 46 sp. From these 
numbers it will be seen that the collectors have not been 
idle, and, no doubt, Mr. Armstrong’s catalogue, with its 
minute index to localities, will be the means of sending 
many to hunt in quarters which they have not already 
visited. Let us hope that they will note something of the 
conditions under which the fossils are distributed, and not 
content themselves simply by bringing away good bags 
full. Collectors cannot be too often reminded that it is of 
more importance, in the interests both of natural history 
and geology, to know one limited district thoroughly, 
than to go roving over half a country merely for the pur- 
pose of picking up finely preserved specimens. Each 
should mark out for himself some practicable area, and 
make it his endeayourto search every bed, eventhe most un- 
promising, noting not only the fossils he meets with, but 
the character of the stratain which they occur. He should 
also observe what effect a change in the character of a 
bed has upon the fossils it may happen to contain; whether 
they increase or decrease in numbers, whether they indi- 
vidually gain in size or become dwarfed, and, should cer- 
tain species disappear, what cthers, if any, are substituted 
for them. It is only by marking carefully such points as 
these that we can ever hope to acquire an adequate con- 
ception of the natural history of the old carboniferous 
lands and seas. Mr. Young is quite sensible of the short- 
comings of the collectors in this matter, and gives them 
some seasonable advice, which it may be hoped they will 
take to heart. If collectors paid better heed to these 
