WAT TRE 
* Vil as 
THURSDAY; APRIL: 18, ~ 1907. 
METEOROLOGICAL OPTICS. 
Meteorologische Optik. By Prof. J. M. Pernter. 
In three parts. Pp. 558. (Vienna and Leipzig: 
W. Braumiller, 1902-6.) 
HE subject of ‘‘ meteorological optics’? has 
searcely yet reached recognition as a distinct 
branch of physical science, as one of which it were 
desirable that the phenomena should be systematically 
gathered together in order that the interaction of all 
the circumstances ailecting any one group of appear- 
ances might be the more clearly appreciated. The 
various matters which may be legitimately included 
under such a title have, individually, not failed of 
their full share of attention and discussion—in the 
nature of the case they have from the earliest times 
been the subject of curious, though 
careful, observation and speculation, 
the phenomena an explanation, at any rate approxi- 
mate, has already been satisfactorily given. The 
cause of the blue of the sky and of the ‘‘ twinkling ”’ 
of the stars, the theory of mirage, of the rainbow, 
of halos and mock suns, of all these much has been 
written, and by many physicists of eminence, but an 
exhaustive classification of all the questions which 
fall within the domain of meteorological optics has 
perhaps hardly been attempted. 
From the optical side, the most complete account 
of- the subject previously given to be found in 
Maseart’s ‘“* Traité d’Optique ’’; but the accumulation 
of meteorological data has proceeded somewhat more 
rapidly of recent years, and it is to the meteorologist 
that we must look for the exact quantitative solution 
of many problems in which the optical theory has 
already been correctly indicated. Prof. Pernter has 
every qualification for the task which he has set 
himself, and as director of the Imperial Institution 
in Vienna for the study of meteorology and terres- 
trial magnetism he brings to his subject an 
acquaintance with meteorological data such as few 
can claim. His book will rank as a classic of scien- 
lifie literature, and is little likely to be superseded, 
within more than one generation, as the standard 
work on this branch of natural science. 
Prof. Pernter explains that his work is based on 
lectures delivered during a period of ten years in 
the Universities of Innsbruck and Vienna. It is, 
however, singularly free from the defects one is pre- 
pared to find in a volume so produced; it is neither 
too diffuse nor too exclusively technical, and while 
essentially scientific, in the strictest sense, in its aim 
of giving an exact numerical explanation of the 
phenomena recorded, it furnishes a _ descriptive 
account of the appearances dealt with, from . the 
records of observations, old and new, which the least 
scientific reader can scarcely fail to find attractive. 
To the contents of the four sections into which 
the subject is divided the author’s own classification 
is the best guide. The first section deals with the 
apparent forms of the vault of the sky and the pheno- 
mena connected therewith—over-estimation of the 
NO. 1955, VOL. 75] 
not always 
and of most of 
is 
heights of mountains, the apparent variation in size 
of sun, moon, and constellations between horizon 
and zenith, &c. In section ii. are included all the 
phenomena which owe their origin to the gaseous 
constituents of the atmosphere, whether 
normal or abnormal conditions—astronomical refrac- 
tion, depression of the horizon, mirage, the “* Fata 
Morgana,” and the scintillation of the stars. In the 
third section are considered the effects due to the 
presence in the atmosphere of masses of particles the 
appearance of which is intermittent—clouds, whether 
of ice crystals or rain-drops; and here we find a full 
discussion of halos and parhelia, coronz and rain- 
The fourth section, which will be issued next 
autumn, will treat of the phenomena due to the 
existence in the atmosphere of very minute particles 
of whatever nature which are always present, but 
which are especially numerous at certain times, as 
after volcanic eruptions. The classification thus in- 
dicated is both natural and convenient; with but rare 
exceptions it brings together all those phenomena 
which and therefore demand 
similar treatment. 
Of the first section it will suffice to say that Prof. 
Pernter gives very careful discussion of the 
apparent form of the ‘‘ vault’ of the sky, basing his 
numerical results especially on the observations of 
Reimann. His conclusion is that the form is that 
of a segment of a circle, the are of which subtends 
at the centre an angle of the order of 40°. From 
this it follows that estimations of dimensions near 
the horizon and at higher altitudes will differ widely, 
a factor having important bearings, as already in- 
dicated, in regard to many every-day phenomena, to 
which we may add the apparently oval form of halos 
and coronz at low altitudes. In his explanation, or 
rather his suggestion, as to the direction in which 
an explanation must be sought, the author follows 
Gauss, who first made experiments to show that 
this subjective effect is mainly due to the normally 
upright position of the body, and to the abnormal, 
or at least unusual, procedure involved in raising the 
eyes from the horizon to the zenith. It need hardly 
be added that there are many points here demanding 
further discussion. 
The second section passes from the consideration 
of the effects due to atmospheric refraction under 
normal conditions to a very full and interesting 
account of the various phenomena due to reflection 
| and refraction when the density of successive layers 
of the atmosphere shows abnormal variations or 
when the density in any region is subject to rapid 
fluctuations. Prof. Pernter is perhaps here at his 
best. For the descriptive portion of the work he has 
sought the most typical examples to be found in 
scientific literature, giving in the words of the actual 
observers the details the explanation of which he 
afterwards follows out as closely as possible from 
the most exact data obtainable relative to the vari- 
ations of atmospheric density. We may direct atten- 
tion especially to the author’s theory of cases of 
exceptional ‘‘ visibility ’’ of distant objects, apparent 
j nearness and magnification. His theory of the 
Cie 
under 
bows. 
are of similar origin, 
a 
