Dec. 21, 1882] 
NATURE 
173 
The Microscope and some of the Wonders it Reveals. By 
Rev. W. Houghton, M.A., F.L.S. Fourth Edition. 
(Cassell, Petter, and Galpin.) 
IT seems sufficient to notice the appearance of the fourth 
edition of this little volume, which, like so many works 
issued by the same firm, bears no date of its appearance. 
The Flora of Essex County, Massachusetts. By Joun 
Robinson. (Salem, 1881.) 
THIS enumeration of the plants of Essex county em- 
braces, besides the Phanerogams, the Vascular Crypto- 
gams, and the alge (marine) and lichens among the 
Thallophytes. Essex County would seem to be an attrac- 
tive field to the botanist. Besides open country, deep 
woods and numerous swamps, the Merrimac furnishes a 
fine fertile valley. The freshwater ponds, over fifty in 
number, are from four to four hundred acres in extent, 
and are rich in water-plants. A sub-alpine flora is to be 
met with, while a long sea-coast affords suitable dwelling- 
places for a large number of plants peculiar to such 
quarters. To this well compiled flora an interesting 
series of sketches of the lives of some of the early 
botanists of the district— Cutler, Osgood, Oakes, Pickering 
—is attached. 
Catalogue of the Fossil Foraminifera in the British 
Museum (Natural History). By Prof. T. Rupert 
Jones, F.R.S. (London; Printed by order of the 
Trustees, 1881.) 
Tue Foraminifera which are in a living state to be found 
widely distributed in the seas of the present day, are also 
known to enter as fossils into the composition of several 
of the stratified rocks, forming in some places such vast 
thickness of limestone, as to command the attention of the 
Palzontologists. It is found somewhat difficult to draw 
the line between recent and fossil forms; and it would 
seem to be equally difficult to be sure what is a foramini- 
ferous form and what is not. In this most useful cata- 
logue, however, all descriptive details and all contro- 
versial questions are omitted. Eozoon appears in the 
list, and so also does Orbitoides. The classification 
adopted is that of H. B. Brady, and the species are 
grouped according to their local occurrence and geological 
age. 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents. Neither can he unaertake to return, 
or to correspond with the writers of, rejected manuscripts. 
No notice is taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible, The pressure on his space is so great 
that it is impossible otherwise to ensure the appearance even 
of communications containing interesting and novel facts.] 
The Aurora and its Spectrum 
In the recent correspondence in your columns on the subject 
of the aurora, no notice has been taken of an old observation by 
Anjou, in Siberia, that whenever the aurora flasbed up past the 
moon, a halo was formed. This, with numerous other observa- 
tions, which need not be detailed here, have let me to the con- 
clusion that suspended crystals of ice have most probably 
something to do with the aurora ; and my object in writing is to 
suggest to some of your readers who are well equipped with 
suitable apparatus, that if they could contrive to pass a glow or 
phosphorescent discharge of electricity through fine-falling or 
loosely-compacted snow, they might very possibly be rewarded 
by the discovery of the origin of the green and red lines in the 
aurora spectrum. 
Mr. Capron’s experiments seem to show conclusively that it is 
not an air spectrum, and it is also evident that the conditions of 
discharge in an atmosphere laden with ice crystals are very 
different from those in the clean vacuum tubes usually employed 
by experimenters. 
While on the subject perhaps I may be permitted to add one 
small contribution to the question. I have examined most of 
the auroras recorded by the Meteorological Office during the last 
four or five years with reference to the synoptic conditions of 
pressure with which they are associated. ‘The result is, that 
though the larger number may be grouped round a few types of 
pressure distribution, it is not easy to see any one constant 
condition. RaLPH ABERCROMBY 
21, Chapel Street, London, S.W., December 18 
Swan Lamp Spectrum and the Aurora 
Mr. J. RAND CApron’s experiment with the Swan lamp is 
very interesting ; but his infernce that the aurora may not be 
an electric discharge in the upper atmosphere because it does 
not show nitrogen lines in the spectrum is hardly justified by the 
experiment. On the contrary, the true significance of that ex- 
periment appears to be that there is a certain degree of rarefac- 
tion of the air (or vacuum) at which the nitrogen lines disappear, 
Such a vacuum is given by the Swan, and probably other electric 
incande:cence Jamps. According to Mr. Capron’s result, when 
more air got into the bulb and vitiated this fine vacuum, the 
nitrogen lines appeared. We may say, then, that if the aurora 
is an electric discharge in the upper air, the rarefaction must be 
approximately that of a Swan lamp, if there are -no nitrogen 
lines visible in the spectrum of the light. To study this further 
some one ought to examine the discharge in vacuum tubes con- 
taining air at different degrees of density. J. Munro 
West Croydon, December 18 
The Meteor of November 17 
Mr. CAPRON’s letter (p. 149) gives an interesting confirmation 
of the meteoric nature of the light seen on November 17; as 
showing that it is physically inpossible that it can be an aurora, 
according to accepted theories of that light. Sett ng a:ide the 
impossible estimate of forty-four miles, it should be noticed that 
the heights assigned are in close agreement, 170 miles being 
merely stated, like other elements in my letter, as a minimum. 
The oblique direction of the metecr from 10° altitude in due 
east to horizon in due south-west, as shown by several observa- 
tions, is another evidence of ils extra-terrestrial origin. 
Bromley, Kent Vive Rule TRG EK 
Invertebrate Casts 
THE communication in NATURE, vol. xxvii. p. 46, induces 
me to state the following fact. Engaged this summer in an 
economic survey of the North Transcontinental Survey for the 
North Pacific Railroad in the camp just opposite Umatilla, near 
the Columbia River, Washington Territory, I observed, on June 
26, the nympha of a new species of Ophiogomphus, then very 
common, emerging out of the water for transformation, The 
Columbia River had been very high, the water beginning to 
recede, was still more than 30 feet higher than usual. The 
country around the camp belonged to the so called sagebrush 
desert, but near the river was a bank of wet sand, flat and 
smoothed by the receding water. There were no 
plants around, and only one willow tree, now about 
1co feet distant from the river, for five miles on one 
side and twelve on the other side. I had observed 
before on the sand a number of traces like the 
diagram. In the middle a straight furrow, and on 
each side two series of equidistant dots. By chance 
I was able to discover that these tracks are made 
by the nympha of Ophiogomphbus (family Gomphina 
in Odonata). The straight furrow is made by the end 
of the abdomen, which is heavy and slides upon the 
ground. The forelegs are shorter, and make, with the end of 
the tibia, the inner <eries of dots. The other legs are longer, 
and make the outer series. More remarkable was it that the 
furrows were made in a straight line from the water to tree, as it 
is scarcely probable that a nympha so near its transformation 
can see well at a distance of about 100 feet. Nevertheless I 
caught the nympha just at the end of the track—which I saw 
made—in ascending the tree. The two outer series of dots 
are one inch distant one from the other. I remember having 
seen an account of similar tracks on fossil slabs, but I have not 
been able to find the publication. H. A. HAGEN 
Cambridge, Mass., November 27 
