August 6, 1885] 
NATURE 
Bet 
and the conjoint Observatory, will be completely executed 
at the close of the year 1887. With its unparalleled in- 
strumental equipment, and an unusual endowment for 
the prosecution of astronomical research ; located where 
the sky is cloudless most of the year, and at such an 
elevation as to be above the clouds a great part of the 
remainder; and situate in a region, too, where the 
steadiness of the air permits astronomical measure- 
ment of the highest precision to proceed uninterruptedly 
throughout the entire night for months at a time,—the 
Lick Observatory is destined, under prudent manage- 
ment, to take its place at once in the foremost rank ; and, 
although it is the first established mountain observatory, 
it may well expect to hold its own in the emulation of 
similar institutions which may subsequently be inaugur- 
ated at greater elevations. 
TWILIGHT} 
ees essay, an extract from a more comprehensive 
work on the problem of twilight, which the author 
hopes to conclude in the course of this year, and embodying 
a lecture recently delivered by him both in Hamburg and 
Leipzig, describes the phenomena of twilight in general 
and of the remarkable sky-glows of the winter of 1833 in 
particular, with clearness, fullness, and exactness, and 
explains the physical causes of these phenomena from a 
special and mature study of that universally interesting 
field of observation, by numerous highly pertinent and 
illustrative experiments, and altogether in a manner which 
should bring home, even to the unscientific reader, a new 
sense and a new intelligence of the painting offered anew 
every morning and evening to the study and delight of 
man universally. 
After relating and taking measure of the stupendous 
outburst of Krakatoa and the brilliant glows involving 
nearly the whole earth for a long period after that event, 
and comparing these two consecutive phenomena with 
the analogous phenomena of the outburst of “ Graham 
Island” in 1831, followed by brilliant twilights and 
peculiar blue and violet sun colours, attracting the ad- 
miration, in particular, of Italy, France, and Germany, 
the book addresses itself to the task of investigating the 
physical laws concatening these two apparently hetero- 
geneous phenomena, and why all volcanic outbursts are 
not attended by the same wonderful optic displays. 
While each particle of dust, smoke, or fog causes a 
bending or diffraction of the light, a collective effect, 
comprehending a brilliant development of colours, is pro- 
duced only when all the particles of matter are of equal 
size and are distributed uniformly in space—a condition 
not even most remotely fulfilled in the case of ordinary 
smoke and fog. Diffraction includes the lateral dispersion 
of the light, which is all the more efficient the nearer the 
edges lie to each other, and therefore the smaller the 
particles are, and also the “interference ” of like-coloured 
rays of light. When a red light falls, for example, on a 
fine glass thread or a diamond stroke scratched into glass, 
the shadow will consist not of one thin black line, but of 
a whole system of parallel stripes alternately dark and 
brilliant, z.e. black and red. When, again, a white light 
falls on the diamond stroke, the reflection shows a system 
of parallel stripes glowing in all the colours of the rain- 
bow. In the case of a single line the development of 
colours is indeed so small as to be scarcely perceptible, 
but with many thousand lines of exactly the same breadth, 
and situated at exactly the same distance from one 
another, the reflex image is such that, taken up on a 
white screen, itis visible at great distances. Perfectly 
corresponding is the case with granules of dust. The 
shadow ofa single granule of dust in red light consists of 
* “Die Dammerungserscheinungen im Jahre 1883 und ihre physikalische 
Erklarung.” Von J. Kiessling, Professor am Johanneum zu Hamburg. 
(Hamburg und Leipzig. 1885.) 
a system of concentric rings, alternately dark and redly 
luminous, which are all the broader the smaller is the 
granule. In white light, on the other hand, the shadow 
of the granule consists of alternately dark and bright rain- 
bow coloured rings. If the dust granules are all of the 
same size, then will the like-coloured rings pretty nearly 
coincide, and, in the case of a sufficiently large number of 
granules, the reflex image will be composed of coloured 
rings of great luminousness. If, on the other hand, the 
dust-granules are of different size, then will all the different 
colours coincide, and, according to a well-known optic 
law, the image will be colourless. The image of a dust- 
cloud may, therefore, be rich in colours, poor in colours, 
or colourless, according as the particles of dust of which 
it is composed are of the same or of different size. 
The experiments of Coulier and Mascart, extended by 
Aitkin, have demonstrated that in a perfectly moist air, 
no formation of fog is possible, however much the tem- 
perature is lowered, so long as the air is absolutely free 
of dust; and that the more air, sufficiently moist, is 
charged with such foreign particles, the more intense is 
the formation of fog under a sufficient lowering of the 
temperature or pressure of the air. Let filtered and com- 
pletely moist air ina glass ball have its pressure dimi- 
nished, then will only a few particles of fog reveal them- 
selves to the most careful inspection, even under the 
powerful light of an electric lamp—particles of fog which, 
moreover, yield not the slightest coloured image. Admit 
now into this filtered air a few cubic millimetres of ordin- 
ary house air, then will a very fine, silvery, transparent 
fog at once form itself, of such slight density that even in 
the case of a considerable area of it the transparency of 
the atmosphere would be but very little affected. At the 
first moment of its formation let a reflected image of the 
sun, or the reflected light of an electric lamp, be viewed 
through it: the image will be seen surrounded by an in- 
tensely luminous blue or greenish light, with a broad, 
reddish ring, the colouring of which may range through 
all stages from brilliant purple red to the most delicate 
pale pink. 
The phenomena of colour produced and explained by 
experiments of the above description are made to serve 
as the key to the more extensive but essentially identical 
phenomena composing the total process of twilight, which 
is distributed, like a spectacular play, into three acts with 
a prelude, and sometimes, though comparatively seldom, 
an afterlude—parts which, however, are not strictly dis- 
tinguished in time, but occur to some extent simulta- 
neously and overlap each other ; as also to the compara- 
tively unimportant deviations—apart from the intensity 
of colouring—from the normal course, which obtained in 
the remarkable sky-glows that arrested universal atten- 
tion throughout the fall and winter of 1883, 
HENRY MILNE-EDWARDS 
E ENRY MILNE-EDWARDS was born at Bruges 
in October, 1800. Having completed his elementary 
studies in Belgium he attended medical lectures in Paris, 
where he took his diploma in medicine in 1823. While 
he retained an interest in medical and surgical pursuits 
until late in life, and was a member of the Academy of 
Medicine, Paris, of the Medical Societies of London, 
Edinburgh, &c., his earliest passion seems to have been 
for the study of natural history, and he soon abandoned 
the practice of his profession and devoted himself to 
scientific researches among the lower forms of animal 
life. 
During the years 1826 and 1828, in company with his 
friend and fellow-labourer Audouin, the assistant to 
Lamarck and Latreille, he made a careful study of the 
various invertebrates to be met with on the coasts at 
Granville, around the Isles at Chansey, and as far as 
Cape Frehel. A member of the French Academy was, 
