May 1, 1902] 
NATURE 
America. The species are divided into six sections, and the 
synonymy, variation, distribution, &c., of each species are 
given in great detail, at least in the case.of well-known 
species. The probable evolution and phylogeny of the 
genus are also discussed, and to the latter subject the 
elaborate “Stammbaum” is devoted. The three maps 
show the distribution of various species of the genus. 
Scientific botanists should find much to interest them in 
Dr. Sterneck’s work. 
A Text-book of Insanity. By Charles Mercier. Pp. 
xiv + 222. (London: Swan Sonnenschein and Co., 
Ltd., 1902.) Price 6s. net. 
Mr. MERCIER addresses his little work directly to the 
ordinary medical student, for whom, it appears from the 
preface, there has hitherto been no text-book of insanity 
of moderate compass. For the practical student so clear 
and brief a description of the leading types of mental 
disorder from the pen of a recognised authority will be of 
high value. The work hasalso its merits from the stand- 
point of the theoretical psychologist, though he will 
probably prefer to study the author’s views in his larger 
work, “ Psychology Normal and Morbid.” The account 
of normal mental activities by which the description of 
insane deviations from the normal is preceded is 
eminently clear and judicious. The psychologist 
should also be thankful to the author for discarding the 
bewildering nomenclature of #zanias and phobias, and | 
offering a simple and intelligible classification of mental 
diseases, based on 
insanity (z.e. the aggregate symptoms presented simul- 
_ taneously at any stage by a patient) and varieties of 
insanity (ze. specific types of the course run bya case 
from first to last). Besides purely medical and psycho- 
logical information, the book contains some useful re- 
marks on the legal responsibilities of the practitioner in 
connection with insane patients. Noe Ee ls 
Lecons sur les Séstes & termes positifs. Par Emile Borel. 
Recueillies et rédigées par Robert d’Adhémar. 
Pp. vili+94. (Paris: Gauthier-Villars, 1902.) Price 
fr. 3°50. 
THIS appears as the third instalment of Prof. Borel’s 
lectures on the theory of functions. It is somewhat more 
fragmentary than its predecessors, and has, in fact, the 
typical qualities and defects of a set of lecture-notes. 
As an introduction to the memoirs of Hadamard, Mittag- 
Leffler and Poincaré, as well as to those of Prof. Borel 
himself, these chapters will be very serviceable. Perhaps 
the most noteworthy articles are those which deal with 
the theory of increment (cvozssance) ; it is there shown 
that there is no natural scale of orders of magnitude. In 
fact, an aggregate of orders of increasing functions can 
be constructed which is not numerable. Moreover, 
functions have been invented which have no regular 
order of increase ; thus an example is given of a function 
which is comparable with exf x for an infinite number of 
values of the variable, and with exp (exp x) for another 
infinite number of values. This will cause searchings of 
heart in certain quarters, no doubt; even Prof. Borel 
remarks that “fort heureusement, les fonctions qui se 
présentent aturellement aux géométres sont, en général, 
de nature plus simple.” 
222. 
H. E. Hadley, B.Sc. Pp. xii + 232 
millan and Co., Ltd., rgor.) Price 2s. 6d. 
THIs is an excellent collection of laboratory experiments, 
suitable for the higher classes in secondary and public 
schools. Magnetism is taken first, then electrostatics 
and current electricity. An appendix gives some in- 
struction for making the necessary apparatus. The 
author wisely confines the experiments to those which 
can be performed with quite simple apparatus. 
NO. 1696, voL. 66] 
the distinction between forms of | 
LETTERS TC THE EDITOR. 
The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake 
fo return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice 2s taken of anonymous communications.| 
A Remarkable Lunar Halo, 
On the night of January 19 of this year a singular lunar 
phenomenon was visible here. The sky had clouded over and 
was covered with a nearly uniform whitish sheeting of cloud, 
through which the brighter stars could be seen. There was no 
wind. The barometer stood at 29°20 inches and the tempera- 
ture was 28° F. The moon, which was near the meridian, was 
ten and a quarter days old and had a north declination of 19°. 
Surrounding the moon was the ordinary lunar halo of 45° or 
50° in diameter, which is so often seen at the approach of bad 
weather. This ring was clearly defined on its inner edge, 
which was of a reddish or brownish colour; it rapidly diffused 
on its outer edge and was perhaps a couple of degrees in thick- 
ness. The whole interior of the ring was darker than the sky 
outside of it anywhere, which is its customary appearance. 
Cutting exactly through the moon, with its centre near the 
Capelly 
° 
e Caster 
Peliux 
SASSI a] 
Fic. r.—Lunar Phenomenon 1902, January 19, 9 p.m. 
zenith—in the region of Capella—was another ring of apparently 
the same dimension and brightness, and similar to the other in 
every respect. It too was sharply defined on its inner edge, 
where it was fringed with a reddish or brownish colour, The 
general colour of the two rings was whitish, with a suggestion 
of yellow. The interior of this ring was also darker than the 
sky outside. There was no noticeable increase of light where 
: ‘ 4 | the two rings intersected. They seemed to merge into one 
Practical Exercises in Magnetism and Electricity. By 
(London: Mac- | 
another without any evidence of the crossing. 
This phenomenon was first seen at 8h. 50m. (6h. om. slow of 
Greenwich Mean Time). It was perhaps visible for some time 
before this. I had been observing with the large telescope 
when the increasing cloudiness had stopped work. It was 
noticed (a few minutes before seeing the phenomenon) that the 
seeing had suddenly got excessively bad. 
The extra ring remained visible until 9h, 20m., at which time 
it disappeared—not all at once, but gradually and unequally. 
During the time it was under observation, from 8h. 50m. to 
gh. 20m., this ring revolved eastward in position angle, about 
