280 
correct. Mr. Rendle, with a former pupil of mine, Mr. W. 
West, jun., has described as new for Britain a variety of 
the genus from a canal in Manchester, where it was assuredly 
introduced (see Journal of Botany, vol. xxxvii., 1899, p. 289). 
I take his word as final. Mr. Minakata may be referred to the 
above-cited paper for the answer to: his second question. P. 
Kewensis must have also been introduced, as it has never been 
found again.” 
Pror. T. Levi Civita has contributed to the Azma/les of the 
Faculty of Sciences of Toulouse a paper having an important 
bearing on the recent discussions as to the production of a 
magnetic field by moving charges. In a previous paper on that 
subject, Prof. Righi had examined the possible sources of error 
in various experiments, from those of Rowland down to the 
recent observations of Cremieu and Adams, and had pointed 
out that some uncertainty was introduced by the presence of the 
conductor used to shield the magnetic needle from electrostatic 
action. This remark has led Prof. Levi Civita to undertake a 
mathematical investigation of the effect of an infinite plane- 
conducting screen on the magnetic field produced by an electro- 
static charge moving uniformly parallel to the plane. The 
results which are embodied in the present paper show that if a 
is the ratio of the velocity of translation to that of light, then up 
to the order of a®, the electric and magnetic forces on the side 
of the screen opposite to the moving charge are derivable from 
a potential. The electric force is negligible, while the mag- 
netic force is reduced to a certain fraction, less than one-half, 
of what it would be at the same point if the conductor were 
removed. The magnetic force is not, however, entirely screened 
by the conductor except in the limiting case when the sheet has 
infinite conductivity. 
THE unique construction of the ‘Cooke’ photographic 
lenses, made by Messrs. Taylor, Taylor and Hobson of Leicester, 
has given rise to possibilities of variation of their focal lengths by 
the user, that are both interesting and useful. The replace- 
ment of the back component by a lens of greater focal length, 
increasing the focal length of the objective by about 50 per cent. 
we referred to some time ago, the alternative back lens being 
known as an ‘extension lens.’’ Messrs. Taylor, Taylor and 
Hobson have now formulated a method by which the focal 
length may be reduced. This is effected by unscrewing the 
front component. One complete turn shortens the focal distance 
of a five-inch lens by nearly half an inch. Such a difference is 
of little use with reference to the resulting alteration in the 
scale of the image, though it may sometimes be convenient. But 
when applied as an alternative to the use of rack-work and 
other devices for increasing the distance between the lens and 
the plate for focussing purposes, as in the use of hand-cameras 
for comparatively near objects, this range is ample. Without 
moving either the objective as a whole or the plate, less than 
half a rotation of the front component of an objective of five 
inches focal length will alter the distance of the object that is in 
focus from infinity to three yards. The makers take advantage 
of this fact in a new issue of their lenses, in which a scale is 
engraved on the mount so that objects at infinity, ten, six, four 
and three yards’ distance may be brought into focus by this 
simple means. Within this range the defining power of the 
objective from corner to corner of a quarter plate, using the full 
aperture of 7/6°5, is so little affected that the deterioration of* 
the image at the edges of the plate can only be detected by 
means of a magnifier. The advantages of this method of 
focussing are that it is more simple from a constructional point 
of view than others now in use, saving the weight of those parts 
hitherto necessary simply for focussing purposes, and that as the 
lens and plate may be rigidly fixed in their relative positions, 
there is less risk of instability or misplacement with the con- 
NO. 1707, VOL. 66] 
NATURE 
[JuLy 17, 1902 
sequent deterioration of definition, The same principle is 
applicable when the ‘‘ extension lens” is employed, thus further 
increasing the range of adjustment possible. 
Messrs. R. FRIEDLANDER UND SouN, of Berlin, have 
issued two catalogues of floras, one of European, the other of 
exotic plants. 
ACCORDING to the Report for 1901, the Manchester Micro- 
scopical Society continues to do excellent work, although the 
hon, secretary has to deplore a diminished attendance at the 
meetings. 
Nature Notes for July contains a notice of Mr. E. N. Buxton’s 
efforts for the re-afforestation of a large part of the old 
Hainault Forest, which was deforested about 1850. The cost 
will be about 20,0007. for the Lambourne and Hainault lands, 
and 7000/. for the Grange Hill Forest. It is proposed to ask 
the great City Corporations and the Essex County Council to 
bear the main cost, although much financial help is expected 
from private beneficence and local bodies. 
In a paper published in vol. Ixxi. of the /ournal of the 
Asiatic Society of Bengal Mr. F. Finn notices certain instances 
of what he terms ‘‘abrupt variation ” in Indian birds. Among 
them he notices a not uncommon colour-phase in the ruff, and 
for the birds displaying this peculiarity he proposes the name 
Pavoncella pugnax leucoprora; this, it may be mentioned, is 
not in accordance with modern practice, which restricts sub- 
specific titles to local geographical forms. The author also calls 
attention to a domesticated cock in the Indian Museum, de- 
scribed many years ago by Blyth, which has partially assumed 
the female plumage, and appears to be the only known 
example, at least in India, of such an abnormality. 
THE failure of pea crops forms one of the more important 
items in the Az//etin issued this year by the authorities of the 
agricultural experiment station at Fort Collins, Colorado. It 
was discovered that the soil was permeated with the hyphz of a 
Rhizoctonia, similar to, if not identical with, that which is de- 
structive to potatoes. Peas are more resistant to the attacks of 
this fungus than potatoes, but under certain conditions, such as 
in a heavy soil which holds the water and while the plants are 
young, the fungus gets the better of the struggle. In the case 
of seeds taken from diseased potato plants, treatment with solu- 
tions of corrosive sublimate or formalin has been found to prove 
efficacious, and probably this will also hold good for peas. A 
Rhizoctonia was also found to be the cause of disease on black- 
berries. Injurious effects of spraying apple trees with Bordeaux 
mixture are reported, causing malformation of the fruit. These 
and other pathological effects are illustrated by excellent plates 
produced from photographs. 
CAPTAIN STANLEY S. FLOWER has issued his Report, for 
1901, on the Zoological Gardens at Ghizeh, near Cairo, which 
are now placed under the Public Works Department of the 
Government of Egypt. The Report gives an excellent account 
of the condition and progress of this instituton, which seems to 
have prospered greatly under Captain Flower’s directorship. 
The Gardens, which extend over about 50 acres, are beautifully 
treed and kept up; they are situated at Ghizeh on the left bank 
of the Nile, and are connected with Cairo by tramway, They 
contain living examples of about 700 species of mammals, birds 
and reptiles, and a great variety of plants. The number of 
visitors increases every year, and was 52,711 in 1901. The 
latest additions to the buildings are an elephant house, a lion 
house and a large aviary, besides other smaller structures. It 
is stated that examples of forty-five different species of wild 
birds were observed within the Gardens in 1gor. 
