76 
NATURE 
fe Be) 
[May 1902 
congratulated upon having produced a very clear and 
readable introduction to the study of geology. The 
illustrations, many of which are new, are especially 
excellent, some being from original photographs taken 
by the author during his travels. 
It is only fair to add, however, that while the earlier 
chapters may be read with advantage by all students of 
the science in every part of the globe, the part of the 
book which deals with “historical” or stratigraphical 
geology is quite unsuited for European students. The 
sequence of formation described is that of the American 
continent, and the fossils figured are, almost without 
exception, American forms. This, while fitting the work 
for students on the other side of the Atlantic, makes the 
work of little value, so far as its later chapters go, to 
English readers. 
Elementary Plant Phystology. By D. T. Macdougal 
Ph.D. Pp. xi + 138. (New Yorkand London: Long- 
mans, Green and Co., 1902.) Price 3s. net. 
WITHIN the pages of his elementary text-book Prof. 
Macdougal has collected together a very large number 
of experiments—so large, indeed, that forty-eight labora- 
tory periods do not by any means exhaust the list. A 
certain number of these would be included in an ordinary 
anatomical course, ¢.g. the examination of sections of 
various parts of the plant, of mycorhiza, &c., while 
others are merely bionomical observations. The inclusion 
of these, however, is not so much deprecated, but rather 
the scant treatment which is meted out to some of the 
more important activities of the plant. Respiration is 
practically limited to a few experiments with seeds placed 
in a retort inverted over mercury ; such apparatus pre- 
cludes any but the roughest quantitative measurement. 
Again, no practical form of potometer is suggested, and 
absolutely no mention is made of the movement of proto- 
plasm. Apart from the actual study of the movement, 
the streaming of protoplasm affords a simple indicator 
when investigating the action of anesthetics or of 
neutral or poisonous gases upon the plant. These in- 
hibiting effects are worked in by the author with growth, 
and this makes the experiments more complicated and 
less adapted to measurement. 
These omissions are the more disappointing because 
Prof. Macdougal has the happy knack of giving explicit 
and full directions in a few sentences, and, further, he 
takes every opportunity of throwing out suggestions 
which should lead the student to think for himself and 
thereby obtain a fuller appreciation of the problems with 
which he is dealing. 
Diagramme der electrischen und magnetischen Zustande 
und Bewegungen. By F. W. Wiillenweber. Pp. 64 + 
plates. (Leipzig : J. A. Barth, 1901.) 
Tus book, consisting of ten plates and sixty diagrams 
and descriptive text, is put forward by the author as a 
contribution to the answers to the questions, What is 
electricity? and What is magnetism? The diagrams 
consist of figures representing the lines of force due to 
various distributions of electricity or magnetism ; but in 
no case is there any quantitative representation at- 
tempted. All that we are given is a distribution of arrow 
heads representing the direction of the ether strains on a 
molecule. The diagrams being purely qualitative, there 
is really nothing in the book that a student could not 
put down himself easily, and frequently with greater 
accuracy than the author. The conception of lines and 
tubes of force as treated by Maxwell and Thomson can 
be most useful and instructive, but as they are given in 
the present book they can only result in confusion. We 
are afraid the questions What is electricity? and What 
is magnetism? are no more nearly answered after the 
appearance of this book than before. 
NO. 1699, VOL. 66] 
LETTERS TO THE EDITOR. z 
(Zhe Editor does not hold himself responsible for opinions exe 
pressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended jor this or any other part of NATURE, 
No notice es taken of anonymous communications.) 
A Remarkable Solar Halo, 
On Sunday, August 7, 1898, being in Norway, I was climb- 
ing, with a friend, the upper slopes of the Horntind, above 
Skogstad, in the well-known Valders route between Christiania 
and Laerdal, lat. 61° 15’ 30”, long. exactly 6° E. We had 
reached a height of about 4000 feet above sea level when we 
saw the very remarkable halo of which I send you the photo- 
graph of the copy of a very careful drawing, made on the spot. 
I first caught site of the halo at 11.30 a.m., on lying down for a 
short rest on a large flat horizontal stone, but I have no reason 
to doubt that it had been visible for some time before. 
early morning had been brilliantly fine, the air still, and the 
sun very hot ; about 10.30 a.m. a very light breeze from almost 
due south began to blow, with intervals of dead calm. When 
the halo was seen, the sky was completely covered with a thin 
white haze. There was, however, no rain that day, though the 
weather on the next and succeeding days was not good. The 
sky outside the circles seemed everywhere brighter than inside 
them ; the sun shone through the haze scarcely brightly enough 
to throw a distinct shadow, and his rays aroused no sensation 
of warmth. The inner ‘edge of all the rings was fairly sharp, 
and of an orange-red colour, brightening into yellow, which 
grew paler towards the outer rim, where it faded into a bluish- 
white radiance, which in turn became imperceptibly blended 
with the white misty sky. The width of the rings was from one- 
and-a-half to two degrees. 
I watched the halo until it had completely faded. First the 
ring svpw faded, the other two complete circles remaining 
visible after it had completely disappeared. Next the ring 
FHWV slowly vanished, leaving the small ring cyRx quite 
perfect and bright, and also the luminosity at 44. This last 
looked like a small part of a fourth circle; certainly it was 
curved and convex towards the sun, but of what radius this 
small arc was I am uncertain, but suspect that it was either the 
same, or greater than, the radius of the two big circles. I had 
no accurate instrument with me at the time for measuring 
angles, but the disc of the sun was distinctly visible through my 
neutral-tinted snow glasses, and in estimating the distance s’R 
as subtending at the eyean angle of 19° I do not think there is 
an error of more than a few minutes of angle. The radius of 
each of the big circles must have been, therefore, nearly 44°, and 
that of the small circle about 22°, 
In the illustration, the width of the rings is somewhat 
exaggerated. But by far the most remarkable thing about this 
halo isthe asymmetric position of the sun with respect to the 
rings. With respect to the two large circles this is obvious, for 
the sun appeared to lie on the circumference of one of them, 
and at a point half way (subject to what is said below) between 
the centre and the circumference of the other ; but, besides this, 
I could not persuade myself, though I exerted all my powers of 
The 
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