33° 
NATURE 
[DECEMBER 28, 1916 | 
Though highly complicated in form, they can be 
applied without excessive labour to the more 
simple problems. 
One or two of the more elementary consequences 
of this theory were given by Einstein some years 
ago. A ray of light must be bent in passing 
through an intense field of gravitation; thus a 
star seen close to the limb of the sun during an 
eclipse should appear displaced 1'7” from its usual 
position. The vibrations of an atom must be 
slower in an intense field, so that the lines of the 
solar spectrum should be displaced slightly to the 
red as compared with terrestrial spectra. It has 
not yet been possible to put these predictions to 
a satisfactory test, and it has been left to the 
completed theory to furnish the first opportunity 
of an appeal to observation. In this the new 
theory has scored a most signal success, for it has 
cleared up the most celebrated case of discordance 
in gravitational astronomy. 
From his generalised law of gravitation Einstein 
has deduced that the elliptic orbit of a planet will 
‘rotate in the direction of motion at the rate of 
247° a 
Here V is the velocity of light, and a, T, e are 
the semiaxis, period, and eccentricity of the orbit. 
If v is the velocity of the planet, the amount is 
practically 67v?/V" radians per revolution. For 
Mercury this works out at 43” per century—just 
the amount of the outstanding discordance be- 
tween observation and theory. The theory also 
gives rotations for Venus and the earth, but their 
orbits are so nearly circular that the effect is im- 
perceptible to observation. For Mars, with its 
strongly elliptic orbit, the correction is more 
important, and sensibly improves the accordance 
between theory and observation.” 
It is rather difficult to grasp the fact that the 
same laws of Nature may hold when some bizarre 
system of co-ordinates is chosen. Suppose an 
observer A uses rectangular co-ordinates, and B, 
through some kink in his mind, uses polar co- 
ordinates without realising that he is doing any- 
thing unusual. For A a ray of light can travel 
along the straight line x=constant; but evidently 
it cannot travel along the circle y=constant, which 
is B’s idea of a straight line. The answer is that 
B through his peculiar system of measurement 
will suppose that he is in an intense gravitational 
field; he will calculate the curvature in the ray of 
light produced by this field; and, making allow- 
ance for it, he will find that the light actually 
travels along its theoretical curve (i.e. curve for 
B, but straight line for A). Thus the same general 
laws of Nature are satisfied for B as well as for A; 
and it might be difficult to decide which of them 
radians per revolution of the planet. 
had got hold of the absolute rectangular co- 
ordinates. A. S. EppincTon. 
® The discordance of the perihelion of Mercury (now removed) was nearly 
30 times its probable error. Of the sixteen secu'ar variations of the four 
inner planets, all are now accordant, except the node of Venus, which 
deviates by 4} times its probable error. Among sixteen residuals we should 
expect to find one of three times the probable error, so that the evidence for 
the remaining discordance is not very strong. There are besides unexplained 
variations of the longitudes of the moon and planets, but these are in a 
different category. 
NO. 2461, VOL. 98] 
PLANTS IN HEALTH AND DISEASE. 
Pees WEISS and his colleagues have done 
well in publishing in book-form abstracts of 
their lectures on Plants in Health and Disease. 
The lectures were delivered at the University of 
Manchester during 1915-16, and had for their 
object the giving of botanical guidance to the 
many small gardeners and allotment-holders who 
were, and are, endeavouring by the cultivation of 
their several plots to add to the food supplies of 
the country. 
Although only its friends recognise the fact, 
science in this country suffers from the virtue of 
modesty. Accompanying that quality—as is so 
often the case—is often a certain hauteur. Hence 
it is that those who—only too rarely, it must be 
confessed—come seeking guidance from science 
are often sent empty away. It would be well for 
science, and also for the country, if this attitude 
of aloof detachment were to cease to be habitual. 
It is true that biological science has at present not 
overmuch to offer to horticulturists; but that is 
an added reason why closer relations should be 
established between those who cultivate the land 
and those who study the science of biology. It is 
by such means as those adopted by Prof. Weiss 
and his colleagues that this contact may be best 
established. 
It is no adverse criticism on this little book to 
say that if and when science and practice go hand 
in hand a thoroughly good botany for gardeners 
will make its appearance, and not before. One of 
the results of such a book will be to teach the 
gardener to do scientifically and better what he 
now does empirically and well. The day of pub- 
lication of that perfect book is far off, and botany 
will have not only to grow but to be pruned very 
heavily before it is written. The science will have 
to cast off the pseudo-encyclopedic habit, woven in 
Germany, which it wears. For example, when 
talking to gardeners of the importance of restrict- 
ing water supply in order to.provoke flower and 
fruit formation (p. 2), why not give an account of 
the current method of tomato-growing instead of 
citing examples from ‘“‘tropical climes”? In the 
cultivation of the tomato the check imposed after 
the first truss of fruit is set, the pinching-out of 
side-shoots, the stopping of the leader, and the 
reduction of leaf surface, all have for their purpose 
the regulation of the water supply and the 
reduction of vigour of vegetative growth. Apart, 
however, from the fact that those who are 
habitually engaged in gardening might not infre- 
quently suggest more cogent illustrations of botani- 
cal principles, the general account given in these 
lectures of the life of a plant is admirable. Occa- 
sionally we discover a lapse, as, for example (p. 7), 
the omission to mention the chief virtue of the 
hoe, namely, that by its use water is. conserved in 
the soil. 
The less difficult parts of the book, those which 
1 ‘*Plants in Health and Disease: being an Abstract of a Course of 
Lectures delivered in the University of Manchester (tgt5-16)." By Prof. 
F. E. Weiss, Dr. A. D. Imms, and Wilfrid Robinson. Pp. 143. (Man- 
chester: At the University Press; London: Longmans, Green and Co. 
1916.) Price xs. 6d. net. 
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