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May 23, 1912] 
LETTERS TO THE EDITOR. 
(The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
A Method for the Detection of the Proximity of 
ice at Sea. 
Tuis method has for its basis the varying alteration 
in the electro-conductivity of sea water in the neigh- 
bourhood of melting ice. The conductivity of such 
water is materially reduced, and is dependent on two 
separate factors: first, the fall in temperature, and, 
secondly, the dilution of sea water of high electro- 
conductivity with water derived from glacier ice of 
comparatively negligible conductivity. 
With regard to the first factor, the fall in conduc- 
tivity is approximately 2 per cent. per degree centi- 
grade for every degree below 20° C., and with regard 
-to the second factor, namely, admixture of ice-derived 
water with sea water, the fall in conductivity, as 
ascertained by direct reading with appropriate 
apparatus, is as follows :— 
(Temperature of experiment throughout =17'8° C.) 
Specific conductivity of sea water, as shown by the 
scale of the apparatus used, =42,000 reciprocal 
megohms. 
With a dilution of 1 part of ice-derived water with 
80 parts of sea water, decrease in conductivity is 
I per cent.; dilution of 1 in 50, decrease=3 per cent. ; 
dilution of 1 in 25, decrease=7 per cent.; dilution of 
I in 10, decrease=12 per cent. 
It is obvious that the presence of ice-derived water 
in increasing proportion in sea water will, with a 
continuous self-recording apparatus, show a_con- 
tinuous fall in the electro-conductivity readings, and 
will so furnish presumptive evidence of the approach 
ofice. It is possible, and even probable, that changes 
in the composition of the water would be more trust- 
worthy than changes in the temperature. In any 
case, if the two effects were observed side by side, 
the results of each method would tend to eliminate 
any disturbing factor peculiar to the other, such as 
the presence of fresh estuarial water on the one hand, 
or, on the other, changes in the temperature due to 
other causes than the proximity of icebergs. 
Myer Cop.ans. 
School of Medicine, The University, Leeds, 
May 14. 
Pinhole Images. 
In the last paragraph of his letter in Nature of 
May 2, Mr. Edser alludes to several ways in which 
“‘pinhole”’ images of the sun’s disc may be observed. 
It is not perhaps so generally known that such 
images are often produced in great numbers by the 
reflection of direct sunlight from a glass surface, or 
by its transmission through a glass plate. 
My attention was directed to these images during 
the recent solar eclipse by observing that direct sun- 
light, reflected on to the ceiling of a room from a 
plate of ordinary unsilvered window glass, contained 
numerous overlapping, but well-defined, crescent- 
shaped images of the uneclipsed part of the sun’s 
disc. Similar overlapping images could also be 
traced in the sunlight coming directly through a 
window pane and falling on the floor, but here the 
best results were obtained by first using a mirror to 
NO. 2221, VOL. 89] 
NATURE 
| reflect the light, 
205) 
after having traversed the window 
pane, on to the ceiling. The mirror, it should be 
said, played no part in the production of the images. 
These phenomena are not observable with perfectly 
flat glass, but only with the common kind of window 
glass, which has a noticeably irregular surface, and 
appreciably distorts the details of objects seen through 
it. For the most part a plate of this glass scatters 
the transmitted light, but here and there, distributed 
over its surface, are small isolated patches which can 
be regarded as truly plane-parallel. In the trans- 
mission of light these isolated patches act as “holes” 
relatively to the surrounding and light-scattering 
parts of the plate, and thus give rise to ‘ pinhole” 
images. The images noted in the reflected light are 
obviously produced in a similar manner by regular 
reflection from any perfectly flat small patches 
scattered over a surface otherwise irregular. The 
uniformity in size of the images and their measured 
dimensions are in accordance with this explanation 
of their origin. R. Beattir. 
Manchester University, May 14. 
Meteor-showers towards the End of May. 
Tue following meteor-showers become due during 
the last week in May :-— 
Epoch May 23, 16h. 30m. (G.M.T.), twenty-first 
order of magnitude. Principal maxima, May 24, 
23h. 15m., and May 26, 20h. 5m.; secondary maxi- 
mum, May 24, 7h. 20m. 
Epoch May 27, 6h., third order of magnitude. 
Principal maximum, May 25, toh. 30m.; secondary 
maximum, May 26, 4h. 5m. 
Epoch May 29, 23h. 30m., twenty-fourth order of 
magnitude. Principal maxima, May 26, 4h. sm., and 
May 28, oh. 50m.; secondary maxima, May 27, 
3h. 1om., and May 29, 13h. 50m. 
Epoch May 29, 19h. 30m., thirteenth order of 
magnitude. Principal maximum, May 31, toh. 35m.; 
secondary maxima, May 20, 2th. 35m., and May 31, 
18h. 20m. 
May 20. Joun R. Henry. 
THE BRITISH SCIENCE GUILD. 
HE sixth annual meeting of the British 
Science Guild was held at the Institution of 
Electrical Engineers on Friday last, May 17; 
and was followed in the evening by a banquet, 
which was attended by a large and distinguished 
company, in the Galleries of the Royal Institute 
of Painters in Water Colours, with the Right 
Hon. Sir William Mather in the chair. Sir 
Norman Lockyer, chairman of committees of the 
Guild, was unfortunately prevented by ill-health 
from being present at either function. His 
absence from the banquet was exceptionally dis- 
appointing, as the day was his seventy-sixth 
birthday, and arrangements had been made to 
mark the appreciation of the members of the 
Guild of his services to science in general and the 
Guild in particular by a presentation of plate to 
him, and a separate token to Lady Lockyer in 
recognition of her energetic work for the Guild 
as honorary assistant treasurer and in other 
ways. 
The commemorative gift to Sir Norman coa- 
