166 
NATURE 
| DECEMBER 13, 1906 
AMERICAN GOOSEBERRY MILDEW. 
M R. E. S. Salmon, the mycologist to the South- 
4 Eastern Agricultural College at Wye, Kent, is the 
active leader of an agitation against a mildew affecting 
gooseberries known as Sphaerotheca mors uvae. ‘The 
fungus in question is of American extraction, and some- 
how it was introduced into Ireland about the year 1900; it 
has since spread, has already effected much mischief, and 
will undoubtedly cause more. 
In Sweden it has, we believe, been very destructive, and 
quite recently its presence has been detected in England. 
Curiously enough, its introduction into Britain has been 
associated with the yellow-flowering currant Ribes aureum. 
This is a Californian shrub that has long been cultivated 
for ornamental purposes in this country, and, up to this 
time, we have not heard of its being subject to the attacks 
of mildew. 
It now appears that Continental growers of gooseberries 
make use of Ribes aurewm as a ‘‘ stock ’’ whereon to graft 
the common gooseberry. Standard gooseberries are by no 
means in general cultivation in this country, and we are 
informed that the use of Ribes aureum is being discon- 
tinued owing to the circumstance that it produces objection- 
able suckers. We do not know what object cultivators 
had in using Ribes aureum as a stock, and indeed we 
were not aware that it was so used until recently, but from 
the circumstance already mentioned that the golden-flowered 
currant has long been cultivated here without detriment to 
neighbouring gooseberry bushes, we may acquit it of any- 
thing but indirect participation in the spread of the 
mildew. 
Still, in whatever way the pest may have been intro- 
duced, we cannot but look upon it as a serious matter. 
The trade in ripe gooseberries is no doubt relatively of 
little importance, but the prices obtained in the market 
for “‘ green gooseberries’’ early in the season are often 
very high, and the market-gardener who was deprived of 
this source of income would suffer seriously. In face of 
these facts it is recommended that the importation of all 
gooseberry bushes, especially of those ‘‘ worked on’? Ribes 
aureum, be prohibited, and that all bushes known to be 
affected should forthwith be destroyed by fire. 
Tt is evident that such measures could never be 
efficiently carried out by individuals. If one grower in 
any district proved negligent, all the careful ones would 
suffer from his default. No system of Government inspec- 
tion would be sufficient to keep out the intruder. Not 
the keenest mycologist in the world could guarantee that 
no fungus spores were introduced even if the importation 
of gooseberry bushes were prohibited. Not the most ex- 
perienced microscopist could guarantee that all the fungus 
spores in a particular plantation were destroyed by the 
cremation of the affected bushes. The spread of the 
Phylloxera throughout the vine-growing countries, despite 
the most elaborate precautions, shows how ineffective those 
precautions were. At the same time they caused much in- 
convenience and loss to the traders and others—a_ loss 
which was all the more serious, as, except in the case 
of vines, it was wholly unnecessary. 
It is to be hoped that if legislation on the lines pro- 
posed by Mr. Salmon be carried out it will be adminis- 
tered with due discretion, otherwise the remedy may 
prove more injurious to the interests of the cultivator than 
the mildew itself. In the meantime the Board of Agri- 
culture has issued a circular giving a description of the 
fungus and of its mode of life, directing the attention of 
Srowers to the imminence of the danger, and recommend- 
ing that every precaution be taken in the purchase of the 
bushes, especially those from Ireland and the Continent, 
that all affected shrubs be forthwith burnt, and that, as 
a measure of precaution, spraying with Bordeaux mixture 
be carried out during the winter in the case of plants in 
any way open to suspicion. The Board states that there 
1s at present no law dealing with the eradication of the 
pests of fruit trees, and that it depends very largely on 
the action of the dealers and of the growers whether or 
not the further development of the pest can be prevented. 
Since writing the foregoing remarks we notice in the 
Times of December 8 that Mr. Massee has, at the request 
of the Board of Agriculture, visited the neighbourhood of 
NO. 1937, VOL. 
id 
Hf) 
Evesham, where he was told that the disease had existed 
for thirty years, ‘‘ and had not affected the fruit,’’ so that 
there is absolutely ‘‘ no necessity for panic.’’ Panic and 
discretion are at opposite poles. If we might offer advice 
to the gooseberry growers it would be that they should 
practise watchfulness and act with discretion. 
RECENT PROGRESS IN MAGNETO-OPTICS.: 
Rotation of the Plane of Polarisation close to an 
Absorption Band.* 
4 ARADAY’S rotation of the plane of polarisation is ex- 
tremely small in all gases, also in sodium vapour. 
Only within a very narrow range close to the sodium lines 
the rotation is positive and very great, a fact discovered by 
Macaluso and Corbino.* In a recent extremely interest- 
ing paper Prof. Wood has given measurements of observed 
rotation of four complete revolutions.* This, however, was 
in rather dense vapour, at least dense in comparison with 
the vapour used in the experiments now to be described, 
in which vapour containing about one-millionth gram of 
sodium per cm.* was used. 
The magnitude of the rotation close to the sodium lines 
is illustrated by measurements made by Dr. Hallo in the 
Amsterdam laboratory. It is clear that on both sides of 
an absorption line the rotation is in the same direction. 
| We may attenuate the vapour still further so that the 
doublet in the direction of the lines of force becomes 
visible. What is the rotation, then, between the compo- 
nents of the doublet ? 
It is easily deduced from Prof. Voigt’s theory that in 
very diluted vapours the rotation must occur, in a sense, 
opposite to that outside the components, and _ therefore 
negatively, and also that it must be very great. In the 
case of sodium vapour I had the pleasure to confirm this 
theoretical result, and to observe rotations of — 400°. 
In these experiments interference fringes in the spectrum 
were used, established by means of a system of Fresnel 
quartz wedges (a method used by Voigt, Corbino, and 
others in similar cases). I will project these fringes on 
the screen. 
If a plate of quartz, which rotates the plane of polar- 
isation, is beld in the ray, you will notice a displacement 
of the fringes. A plate of glass has no influence, of 
course. I have here a quartz plate which rotates the plane 
of polarisation through 90°, and you will notice a dis- 
placement of half the distance between two fringes. A 
displacement of the entire distance between two fringes 
corresponds to a rotation of half a revolution. : 
Analysing the light by means of a Rowland grating, we 
can produce such a system of fringes for all wave-lengths, 
and can consider the rotation for. wave-lengths close to 
the controlling absorption bands. On the screen I will 
first project the fringes close to the sodium lines with the 
field off. The dark vertical lines are the sodium lines. 
They are broad, because the vapour is rather dense. The 
horizontal bands are the interference fringes. With the 
magnetic field on, the image now projected is seen. 
You see how fast the rotation increases in the vicinity 
of the absorption lines, becoming more than 180° closer 
to the bands. In the interior of the bands only a hazy 
fringe is seen. A remarkable equation, first deduced by 
Becquerel,® gives the law of the rotation. The phenomenon 
is more beautiful as soon as the vapour is so thin that 
the doublet is seen (Fig. 5). ; p 
Outside the components of the doublet the fringe rises 
upwards, but inside the components the fringe has moved 
downwards; the rotation is negative there. The rotation 
is —90° for D,, nearly —180° for D,. It is very interest- 
ing to watch the movement of the fringes in the spectro- 
scope as the field is increased or the density of the vapour 
changed. ; 
1 Discourse delivered at the Royal Institution on Friday, March 30, by 
Prof. P. Zeeman. Continued from p. 140. 
2 Zeeman, Proc. Ac. Sciences, Amsterdam, May, 1902. 
Amsterdam, 1902, Archiv Néerl., ser. 2, T. 10, p- 148, 1905- 
3 Macaluso and Corbino, Comptes rendus, cxxvil., p- 548, 1808. 
4 Wood, Phil. Mag., October, 1905. 
5 Becquerel, C.A., cxxv., p. 679, 1897. Cf also Schuster, ‘‘ The Theory 
of Optics,” pp. 291-204, 1904. Siertsema, Proc. Ak. Amsterdam, xii. 
P- 499, 1903. 
Hallo, Thesis, 
