122 
NATURE 
[ Fuxe 16, 1870 
subject, and that “his intention is to offer a new interpre- 
tation of observed phenomena.” The “contemporary 
literature” which has afforded the materials for the “new 
interpretation” consists of Page’s “ Geological Manual,” 
Phillip’s “Vesuvius,” and very many extracts from the 
current weekly and quarterly periodicals for the last year. 
With the exception of an allusion to M. Daubrée, obtained 
from an appendix to Professor Haughton’s “ Manual of 
Geology,” the author shows no acquaintance with any of 
the French or German authorities who have worked so 
hard at his subject. He is, moreover, in ignorance 
of the labours of Sorby, and of the translation of Bischoff, 
published by the Cavendish Society. Had he read Lyell’s 
“ Principles” he would certainly not have been betrayed 
into writing such a strange book as the “ Interior of the 
Earth.” 
We will give one sample of the quality of “the new 
interpretation.” “Page tells us ‘that the interstratified 
trap-tuffs, the basaltic outbursts, and the numerous faults 
and fissures testify to a period of intense igneous activity.’ 
We have a familiar example of this action in our hot-beds 
and our hay-ricks.” Mr. Malet then proceeds to explain 
the earth’s heat and volcanic phenomena by a like action on 
buried vegetable matter. To combat views such as these 
would be absurd. Their author has succeeded in gaining 
a place among geologists similar to that of the circle- 
squarer among mathematicians. Z. 
Sitzungsberichte der Naturwissenschaftlichen Gesellschaft 
Isis in Dresden. (Jahrgang, 1869. Nos, 1o—12.) 
THIS part of the report of the Isis Natural History Society 
of Dresden contains as usual a great number of interest- 
ing notices in all departments of natural history, the most 
important being on botanical subjects, namely, the com- 
mencement of a prodromus of the Lichens of Saxony, 
Thuringia, and Northern Bohemia, by Dr. L. Rabenherst, 
and the conclusion of a synopsis of the Coniferae, by M. 
Laessig. Inthe latter, diagnoses of the families and genera 
are given, and the general characters and geographical 
distribution of the species are indicated. In the continua- 
tion of a lecture on extinct mammalia, Dr. Giinther 
noticed the following species :— Canis familiaris fossilis, 
C. speleus (=C. lupus), C. vulpes fossilis, Hyena spelea, 
Felis spelea, F. antigua, Cuv., . minuta fossilis, Wagn., 
F. aphanista, and ogygia, Kaup, and some species of 
Mustela. \n a paper on recent explorations for rock-salt 
in Prussia, M. Otto noticed the occurrence of a vast bed 
of that mineral near Sperenberg, where a boring has been 
carried down 2,270 feet, of which 1,920 feet is through a 
salt bed. The boring has probably nearly reached the 
bottom of the deposit, as the material brought up now 
contains much anhydrite. Near Segeberg, in Holstein, 
a deposit of salt has been met with at a depth of 400 
feet. 
Mitne-Edwards. Legons sur la Physiologie et V Anatomie 
comparée. Tome ix. 2™ partie. (Paris : Masson et 
fils.) 
WE gladly welcome another instalment of the Jong and 
great work, the “beautiful legacy,” as Bernard has called 
it, of the venerable French naturalist. At a time when 
the “differentiation” of study is carried to such an extent 
that many physiologists know very little about other 
animals than frogs, rabbits, dogs, and men, and many 
zoologists have a very meagre acquaintance with the 
results of experimental physiolegy, such a work as this, 
which skilfully weaves together all the niain facts of 
animal biology, is most wholesome reading. The present 
part continues the discussion of the Reproductive Organs 
of the Invertebrata, and contains two lectures on the 
Development ofthe Embryo. We trust the distinguished 
author may be spared still to preside over and finally to 
witness the conclusion of this great work, 
M.F. 
LETTERS TO THE EDITOR 
(The Lditor does not hold himself responsible for opinions expressed 
by his Correspondents, No notice is taken of anonymous 
communications. | 
The Apparent Size of the Moon 
Dr. INGLEBY is curious to know what Prof. Helmholtz 
would say on this vexed question. If Dr. Ingleby will turn to 
page 630 of the ‘‘ Physiologische Optik,” he will find that Prof, 
Helmholtz has anticipated his wishes. As others of your readers 
may be interested in seeing how the matter is treated by one 
who is facile princeps in this department, I subjoin a transla- 
tion of the passage. If the curious experiment mentioned 
by Dr. Ingleby had referred only to the vertical diameter of 
the disc, it would have seemed to be another illustration of our 
inveterate tendency to ascribe an exaggerated value to vertical 
lines or angles, at or near the horizon, It is said that if ten men 
be required to fix off-hand on a star half way between the zenith 
and horizon, nine, at least, will choose one very much too low. 
If an exact square be cut out in paper and pinned against the 
wall opposite to the eye, the sides will appear longer than the 
top or bottom. If an equilateral triangle be placed in the same 
position, the angles at the base will appear larger than the 
angle at the vertex. If aline be drawn parallel to the bottom 
of a sheet of paper, and a second line, making with it an angle 
of 20° or 30°, any one attempting, without moving the paper, 
to draw a third line through the point of intersection, so as to 
make an angle with the second line equal to that which the 
second makes with the first, will make the second angle too 
large. (This experiment is guaranteed by Helmholtz.) After 
reading Helmholtz’s theory, metaphysicians may be willing to 
allow that all these illusions are to be derived, after his example, 
from the clouds. As metaphysicians have, before now, con- 
tributed a good deal to the clouds, it is perhaps only fair that 
the clouds should contribute something to the metaphysicians. 
W. T. RADFORD 
“*To this category also belongs the celebrated question why 
the moon appears larger when she is near the horizon than 
when she is high in the heavens, although, in point of fact, 
owing to atmospheric refraction, her vertical diameter ought in 
the former case to seem less than in the latter. Even Ptolemy 
and the Arabian astronomers were perfectly aware that the 
true reason why the moon appears larger when seen in the 
horizon, is that she then appears further off. The real ques- 
tion therefore is, why the sky should appear further from us at 
the horizon than it does at the zenith. Various causes have 
been assigned for this fact, and Iam myself disposed to admit 
that there are several causes which combine to produce this 
effect, so that it may be difficult to say which of these causes 
predominates in any one case. 
“First of all we must remember that there is no decisive 
reason why the sterry firmament should appear to us to be a 
spherical surface. It certainly reveals to us objects (the stars) 
which are at an infinite distance ; but hence we can only infer 
that it may assume the appearance of any such indeterminate 
surface as any motive whatever may lead us to ascribe to it. If 
we were floating in empty space, and could survey it in its whole 
extent at the same moment and in all directions, or if its move- 
ments were so rapid as to make a distinct impression on the 
senses, there might be more reason for assigning to it a spherical 
rather than any other kind of surface. In point of fact, however, 
its apparent form and apparent direction are constantly chang- 
ing, according as the portion we happen to see is more or less 
enclosed by various terrestrial objects, and according as we fix our 
attention on a higher or a lower spot. We shall see further on 
that we are naturally disposed to regard it as a plane surface, at 
right angles to the line of sight, whenever both eyes are steadily 
fixed on one point. 
‘*But with the canopy of cloud the case is entirely different. 
The clouds in general are so far from us that the criteria for 
judging of distance which binocular vision or the movement of 
our own bodies can supply are utterly useless. But the clouds 
are often disposed in parallel lines, they generally drift with a 
constant velocity and in the same direction; when near the 
horizon they appear like bars across the sky seen edgewise, 
and so lighted that it is easy to perceive they are bodies whose 
horizontal extension is foreshortened by perspective. All these 
indications serve to give us the impression that the true form of 
the canopy of cloud, at least in the zenith, is that of a very flat 
Araya ipa 
