466 
On the Polarisation of Light in the Rainbow 
As I do not remember seeing any notice, in books on light 
and colour, about the polarisation of light in the rainbow, I 
think it my duty to relate the following facts, although I can 
scarcely think the appearance has been unobserved till now. 
Three times I have tested the rainbow-light this summer, 
and each time I found it wholly polarised. On the first occa- 
sion, while looking at the rainbow, I thought I would examine 
it with a tourmaline, which I kept in my pocket. 
at the bow, through the tourmaline, and saw the bow; but 
on rotating the tourmaline the bow alternately disappeared and 
reappeared at every quarter turn: while the light from a stack 
of chimneys which stood within the bow remained apparently 
unchanged. From this I interred that the light of the rainbow 
was wholly polarised, while the other light in its neighbour- 
hood was not so. 
I have observed the. vanishing and reappearing of the light 
of the rainbow on rotating the tourmaline on two occasions 
since that. I have waited for these additional occasions to 
make sure of the fact, as I was called away from the first ob- 
servation ; and when I could go back the rainbow had vanished. 
The date of the second and third times are August 28 and 
September 4, 1873. 
Leicester, Sept. 5 Gro, FINLAY 
[The polarisation of the light of the rainbow was observed 
by Biot in 1811, and by Brewster in 1812. (See “ Brewster's 
Optics,” art. 185.) With respect to rainbows by reflection, 
there are two kinds—(1) that observed by X. Z. Y., in which 
the light comes to the eye from the water. This is not thought 
worthy of special mention by Brewster. (2) That in which the 
light of the sun reflected from water strikes the shower and 
forms a bow not concentric with the common bow. (See 
“Brewster’s Optics,” art. 186.) It is very easy to see that 
these two kinds of bow form parts of the same cone whose 
axis is at the same altitude as the sun, but in the opposite 
azimuth.—J. C. M.] 
Autumnal Typhoid Epidemics 
THERE appear to be two types of these,—first, the malignant 
and dangerous, which breaks out in isolated spots and is usually 
traceable either directly or indirectly to some sins of sewerage ; 
and a second or milder form, which extends over far larger areas, 
is much more general, and apparently unconnected with sewage 
exhalations or liquid contaminations. Some observations I have 
lately made suggest an explanation of the origin of this latter 
form. We have had just a moist and rather warm summer, 
followed by an unusually wet autumn, Turnips, swedes, beets, 
mangolds, cabbages, potatoes, peas, &c., put forth luxurious 
foliage, and much of this, especially the lower leaves of turnips, 
swedes, and cabbages, have been rotted by the recent rains—so 
much so, that many a country lane that should have exhaled 
sweet balmy odours has been'the abode of most unromantic 
stink. This is especially the case in the flat market garden 
areas that lie by the side of the Thames, and in these the most 
especially where cabbages are cultivated. I have no doubt that 
the partridge shooters of 1873, who have largely availed them- 
selves of the cover of turnip-fields, will confirm my observation 
of their offensive odour. 
Modern agriculture is, in England, chiefly developing and 
extending in the direction of root crops for cattle feeding, and 
the foliage of these is very liable to offensive decomposition 
under the conditions above named. When the autumn is hot 
and dry, their outer leaves, and also those of kitchen vegetables, 
drop off and return to the soil in a dry, crisp, and inodorous 
condition. 
That the moist decomposition of such vegetable matter should 
supply nourishment to disease germs analogous to those which 
are fed by sewage, and that the exhalations of the decomposing 
vegetables should spread them after the manner of sewage ex- 
halations, is obviously probable. 
If I am right, the widely extended and milder forms of autumn 
epidemics should be most prevalent in such years as the present, 
and should prevail more especially in market-garden and cattle- 
feeding districts. 
So far as my own means of observation extend, this appears to 
be the case, but as these are too limited to justify any positive 
conclusion, I throw out the above as a merely suggestive ex- 
planation, demanding further confirmation, which some of the 
readers of NATURE may be able to supply. 
Woodside, Sept. 8 W. Marttigu WILLIAMS 
NATURE 
I looked | 
[ Oct. 2, 1873 
nnn meee Ea ats SESS 
Venomous Caterpillars 
OBSERVING a letter in NATURE respecting venomous cater- 
pillars, I venture to offer a few remarks from personal ex- 
perience. 
The rough hairy caterpillars have a bad reputation everywhere. 
Asa boy, the nurses told me if one got tight round my finger, 7 
(and of course I understood the fingef) would have to be cut off. 
In Switzerland they are regarded by the common people as 
poisonous, though, as far as I know, without foundation. 
In Portugal there is a most remarkable gregarious species, 
known as the “‘ procession caterpillar,” from the great numbers 
that may be seen advancing in a body. This kind has undoubt- 
edly the power of causing very considerable irritation to a 
tender skin. A specimen once crawled up the arm of my little 
girl, then one year old, leaving the skin-surface red and in- 
flamed along its track; and there was a tradition at Lisbon of 
a child that had fallen into a mass of these larvae, and subse- 
quently died from the consequent inflammation. 
In Brazil there is a species in the neighbourhood of Rio that, 
with regard to the formidable nature of its external clothing, is 
a veritable porcupine. It corresponds remarkably with the de- 
scription of the Burmese specimen, both in size and colour. 
The hairs, in a state of repose, are, however, but slightly erect, 
and it is only when irritated- or alarmed that it raises them in 
hostile guise. There can be no question as to the stinging pro- 
perties of these hairs, to which my wife, among others, can bear 
testimony ; but as our experimental ardour did not induce us to 
grasp the creature, the consequences were never serious. The 
largest hairs must be nearly an inch long, and the points of all 
have a lighter appearance, as though singed. It was interesting 
to watch theirelevation by the animal on the approach of the 
finger, as though by some electric attraction. The stinging sen- 
sation is analogous to that caused by a nettle. I am inclined to 
think that in this case the cause was likewise analogous. It is, 
however, possible that the hairs are brittle, or armed with articu- 
lated branches. 
With reference to the power of detaching hairs possessed by 
some caterpillars, a remarkable instance came under my notice 
in Tiguca (Brazil). It was observed in the larva of a beautiful 
black and white butterfly with conspicuous yellow tail. The de- 
termining principle of its existence appeared to be rather economy 
than defence. Consequently the hairs with which its body was 
covered were utilised in the construction of its cocoon. For this 
purpose it was first clearly necessary to shed them ; after which 
they were dexterously crossed and recrossed over the creature’s 
body ensconced under the shadow of some convenient leaf. In 
this process, if thread was used at all, it was with the greatest 
economy. i 
As it was evident that such hairs must be well adapted to their 
purpose, I examined them under a good microscope, when I 
found them armed with short barbs on all sides, especially 
towards the extremities. The spines were tolerably thick, giving 
under the lens much the appearance of a sprig of juniper. 
Berne, Switzerland C. EDEN 
In reference to the article on venomous caterpillars in 
NaTuRE of the 14th inst., I beg to offer you, if the subject is 
not closed, my own very unpleasant experience. 
On the 19th of June last, as I was sitting in my drawing- 
room near an open window, looking on the garden, I suddenly 
felt an itching sensation in my throat and arm, and on examining 
my dress I found a large brown long-haired caterpillar, In a- 
few moments my skin, on the parts affected, was covered with a 
strong eruption attended with intense heat. Thinking it im- 
possible that the insect could have produced this inflammation, 
I sent fora doctor. After examining the skin he assured me he 
could see no other cause, and that the eruption resulted from the 
hairs of the caterpillar remaining in the skin. 
He ordered me some simple applications, telling me that a 
few hours would bring relief. In this he was totally mistaken. 
The inflammation increased to the extent of producing general 
fever ; I passed a sleepless night, and the next day it continued 
unabated. After that it very gradually subsided, but the traces 
of the eruption were visible ten days afterwards, 
The insect could not, I imagine, have bitten me, as I felt 
nothing at the moment. 
Ihave frequently been bitten by tropical insects, but in no 
one instance have I suffered so severely, or been so disfigured. 
The sensation reminded me somewhat of the prickly heat, only 
it was infinitely more intolerable. 
There was no predisposing cause, as I was at the time in good 
fe 
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