56 
simply as a consequence of their great inherent velocity, the 
theory becomes admirably fitted to strengthen the views of 
Olbers, Zollner and Bredichin with regard to the nature and 
the origin of the repulsive force acting upon the cometary 
matter. But the introduction of Maxwell’s pressure of light 
gives rise to a number of difficulties which, as Prof. Arrhenius 
abundantly shows, can only be overcome by arbitrary and un- 
warranted assumptions, 
I shall take an early opportunity of demonstrating the 
superiority of Zdllner’s theory over the one which now claims 
to ‘‘sweep the astronomical horizon of so many mysteries.” 
Royal Observatory, Edinburgh. J. Ham. 
Stopping down the Lens of the Human Fye. 
May I be permitted to direct Mr. Wm. Andrews’ attention 
to the fact that his experiment in ‘‘ stopping down ” the lens of 
the eye involves exactly the same principle as ‘‘ orthoptics,” of 
which every rifle-shot will have had experience. 
The ‘‘ orthoptic ” consists of a round hole in a black disc, 
which replaces the lens of a pair of spectacles. The hole is 
generally adjustable in size, to suit varying conditions of light. 
The purpose of the orthoptic is to increase the depth of focus, 
enabling both back and fore sights and the target to be in sharp 
focus together. Persons with naturally large pupils will, as a 
rule, notice the effect more strongly. H. Buss. 
May 9. 
Ir may interest your readers to know that the principle 
referred to, under the above heading, in your issue of May 8 
was adopted, a great many years ago, by the late Lord 
Sherbrooke, whose sight I believe was very defective. I 
remember seeing, about the middle of the seventies of the last 
century, at an exhibition of physical apparatus at South 
Kensington, a pair of spectacles which were said to have been 
invented by him for his own use. They consisted of two 
convex metal cups, closely resembling in shape and size the 
bowl of an ordinary tea-spoon. In the centre of each cup was 
a small pin-hole, which was the only aperture through which 
light could enter ; and the two cups were fastened together by 
an elastic string, evidently intended to go over the head. The 
invention impressed me at the time as a remarkable example of 
scientific skill combined with great simplicity of contrivance. 
GERALD MOLLoy. 
The Evolution of Snails in the Bahama Islands. 
Ir seems desirable to call the attention of evolutionists to 
Dr. Hi. A. Pilsbry’s monograph of the genus Cerion (or 
-Strophia), just published in the ‘‘ Manual of Conchology.” 
The facts presented are most of them not new, but all that is 
knownis set forth in great detail, with an abundance of excellent 
figures. Cerion is a genus of rather large cylindrical land- 
shells, for the most part inhabiting the Bahamas and Cuba. It 
has split up into innumerable local species and races, 134 of 
which are recognised as sufficiently distinct to bear names. Not 
only do even the smallest islands or ‘‘ keys” produce distinct 
species, but frequently one small island will have two or more 
different forms inhabiting different parts, and sometimes a dis- 
tinct race will occupy a very small area, surrounded on all sides 
by another type. The problem of the differentiation of the 
Achatinellidee in the Hawaiian Islands is complicated by the 
complexity of their environment; but here in the Bahamas we 
have differentiation just as marked, with an environment—small 
sandy islands with palms and low bushes—as simple as we are 
likely to find anywhere. It would therefore seem that an ex- 
cellent opportunity lies before some student of evolution to 
investigate exhaustively the local species and races of these 
Bahama snails, and determine what causes have brought about 
the known results. Colonies could be taken to new localities, 
and watched from year to year to see whether they became 
modified. The food and moisture conditions might be altered, 
and the results observed. The exact conditions surrounding 
each distinct form might be studied and described. Thus it 
might be determined whether the differentiation was the result 
of natural selection or has taken place independently of it. Such 
an investigation would be delightful work for some enthusiastic 
naturalist, especially with such an excellent guide in hand as 
Dr. Pilsbry has supplied. T. D. A. COCKERELIL. 
East Las Vegas, New Mexico, U.S.A., April 26. 
NO. 1698, voL. 66] 
NATURE 
[May 15, 1902 
Retention of Leaves by Deciduous Trees, 
THE retention of leaves by beechen hedges referred to by 
your correspondent in NATURE, April 10, is by no means con- 
fined to those on elevated ground. It may commonly be 
observed in hedges of this tree whatever their situation. In 
Northumberland the beech isnotinfrequently used as a hedge, and 
always retains its leaves throughout the winter. Young beech 
trees also frequently retain their leaves, and by no means always 
in exposed situations. Indeed, the examples I have myself seen 
have been much more frequently in sheltered spots, as in planta- 
tions of older trees. 
Nor is this phenomenon of deciduous trees retaining their 
leaves under certain conditions confined to the beech. It is, 
perhaps, equally common in the oak. Young oak trees in 
plantations may often be seen in the spring covered with brown 
and withered leaves. Larger trees may also sometimes be seen 
retaining the leaves on some of the /owe branches, while the 
upper ones are bare. Travelling from Eastbourne to Victoria, 
soon after reading the above communication in NarurE, I 
noticed hundreds of young oaks covered with withered leaves. 
None of these were in elevated or exposed situations. Indeed, 
I am inclined to suggest, as an inference from the above facts, 
that it is rather the Arofection enjoyed by the trees which exadles 
them to retain their leaves. In the one case the lesser height 
of the tree, and in the other the close intergrowth of the hedge, 
gives the wind less power to strip off the leaves. We can 
hardly consider that there is here a ‘‘ protective device,” unless 
on the part of the gardener who sets a beechen hedge to shelter 
his plants. G. W. BULMAN. 
13 Vicarage Drive, Eastbourne, May 3. 
WITH regard to the interesting communications concerning 
the retention of their leaves by young beeches, I beg to forward 
another possible solution. The beech is a ‘* frost-tender” 
species, and early frosts, which would not rise high enough to 
affect large trees, would freeze the leaves of ‘‘ small young” 
trees, thus preventing the formation of the abscission layer of 
cork at the base of the petiole. In such a case there is no 
reason why the leaves should fall off for a considerable time. 
Leaves killed before the formation of this layer remain on the 
branch for an indefinite time, of which phenomenon pea-sticks 
cut in full leaf may serve as an example. Bevis 
May Io. 
THE RECENT VOLCANIC ERUPTIONS IN 
THE WEST INDIES. 
N EWS of the terrible volcanic eruption in Martinique 
reached this country on Thursday last, and the 
details which have since become known have shown that 
an appalling disaster has occurred. St. Pierre, the chief 
commercial centre of the island, has been totally 
destroyed, and about thirty thousand people have 
perished. The eruption of Mont Pelée began on the 
night of Saturday, May 3, when large quantities of 
scoriae and volcanic ash were thrown into the surround- 
ing country. On Monday, May 5, a stream of lava is 
reported to have rushed down the side of Mont Pelee, 
following the dry bed of a torrent, and reaching the sea, 
five miles from the mountain, in three minutes. When 
the stream met the sea the water receded 300 feet on 
the west coast, returning with greater strength in a large 
wave. , 
Two days later, on May 8, a similar torrent of incan- 
descent lava engulfed the town of St. Pierre. The 
following telegram describing the calamity was received 
at Paris from Fort de France on May 11, and was 
published in Monday’s Zzes. 
“‘The town of St. Pierre was destroyed on the 8th about 
8 a.m. A -terrible torrent of incandescent lava, from Mont 
Pelée, a volcano a few kilometres from the town, accompanied 
by a shower of fire, in a few seconds covered the town, and an 
immense furnace extended over the neighbouring coast, thus 
forming a line of fire from the village of Carbet to the town of 
Précheur. The effects of this volcanic torrent were felt as far as 
