575 
NATURE 
[AprIL 18, 1907 
different forms of mirage, whether due to reflection 
from above or below, or from the side, is essentially 
that given by Tait in his paper ‘‘ On Mirage”? in the 
Edinburgh Transactions, Of the ‘‘ Fata Morgana,” 
two specially interesting examples are cited from 
observations by Prof. Boccara in 1900 and igor, 
while of distortion due to abnormal atmospheric re- 
fraction the most striking cases are those seen by 
Arctowski, as quoted from the records of the Belgian 
Antarctic Expedition. 
The theory of the scintillation of the stars and 
planets, and of the analogous phenomena observed 
in the sun and moon when almost eclipsed, has 
aroused an exceptional amount of attention, and is 
here very fully considered. Lord Rayleigh has dis- 
cussed the matter in a paper on the theory of stellar 
scintillation, where the failure of the “‘ interference ”’ 
theory as propounded by Arago is made clear. The 
various optical effects in which the phenomenon may 
be said to consist, the quivering and the fluctuations 
in intensity and colour, as well as the meteorological 
conditions, the extent and character of the atmo- 
‘spheric ‘“‘ striz,’’ with the best methods of observ- 
ation and measurement, are most completely dealt 
with in the papers by Karl Exner in the Sitzungs- 
berichte of the Vienna Academy of Sciences and else- 
where. Prof. Pernter accepts what he designates the 
Montigny-Exner theory as giving a complete and 
exact explanation of the whole phenomenon. 
The theory of halos, of parhelia and anthelia, and 
allied phenomena given in the third section of the 
book is founded on the classical work of Bravais 
“Sur les Halos,’’ which dates from the middle of 
last century. Though Prof. Pernter speaks of this 
theory, elaborated by Galle and Bravais, as giving 
an entirely satisfactory and in all main essentials 
complete explanation of the phenomena, he is yet 
able, with the greater mass of more exact records of 
observations at his command, and the more accurate 
knowledge as to the form and optical characteristics 
of ice-crystals—six plates of reproductions of photo- 
graphs of various types of ice-crystals are given—to 
confirm or correct in many details the earlier theory. 
The rarity of some of the appearances necessarily 
renders numerical confirmation difficult, and it may 
be well to note, for example, the desirability of further 
careful observations of the ‘‘schiefe Bogen von 
Lowitz,’’ the lateral ares tangential to the halo of 
22°; and of the parhelia related to the halo of 46°, 
especially the colour effects when the sun is in the 
horizon. The whole discussion is fully and clearly 
given, and is very suggestive of the possibilities of 
further meteorological research. 
The author next deals with the phenomena due to 
diffraction effects, whether seen directly by trans- 
mitted light or by reflection—coronz, the ‘‘ Glory ”’ 
or ‘ Brockengespenst,’’ iridescence of the clouds, 
&c.—with an exposition of the theory as developed 
by Fraunhofer, Verdet, and Exner, and based on 
Airy’s development of an expression for the variation 
in light intensity in the diffraction image. The 
volume concludes with a complete discussion of the 
rainbow. Prof. Pernter follows the theory of 
NO. 1955, VOL. 75] _ 
Descartes to the point where its neglect of the con- 
sequences of diffraction leaves it inadequate to explain 
the phenomena, basing his Subsequent development 
on the Airy ‘‘ rainbow-integral’’ for the intensity of 
light in the neighbourhood of a caustic. 
Prof. Pernter suggests that, in a subject of which 
so much has been written in monograph, his work 
must necessarily be of the nature of a compilation. 
He expresses the hope, however, that it may perhaps 
claim to be more than a mere compilation. No one 
who has read his work with any attention will be 
likely to question this claim. Rather it may be 
taken as the model of what a standard treatise on 
a branch of physical science should be, written by 
one whose researches have done very much to remove 
difficulties and to lighten obscurities. Dealing as it 
does with matters of absorbing interest, it is un- 
questionably a bool to be read by everyone who takes 
an interest in the study of natural phenomena. 
THE NEW EVOLUTION. 
Recent Progress in the Study of Variation, Heredity 
and Evolution. By R. H. Lock. Pp. xv+299. 
(London: John Murray, 1906.) Price 7s. 6d. net. 
HE labours of a new school of biologists, ably 
represented in this country by a band of ener- 
getic workers at Cambridge of whom the author of 
the present book is not the least distinguished, have 
been of great service both direct and indirect to the 
study of evolutionary method. It was perhaps to be 
expected that in the first flush of enthusiasm caused 
by the re-discovery of an important generalisation 
like Mendel’s, judgments should be formed and state- 
ments made some of which may seem to pass the 
bounds of scientific caution; but signs are not want- 
ing that a more restrained attitude is beginning to 
prevail, and it is a healthy symptom that the free 
use of the experimental method, rather than mere 
academic discussion, characterises the worl: of the 
new evolutionists. 
A noteworthy point in the biological movement of 
the day is the response that is being given in various 
quarters to the reasonable demand for quantitative 
treatment of the facts of variation, selection and 
heredity. From the side both of the biometricians 
and of the Mendelians, statistical evidence is being 
accumulated and dealt with on a scale that might 
have satisfied Stanley Jevons himself. It must be 
confessed that the pretensions of these two schools 
are at present more or less antagonistic to each other 
and to the convictions of orthodox Darwinians; it is 
certain, however, that the questions raised in the 
course of this three-cornered rivalry are of the greatest 
importance, and that nothing but good can come of 
their thorough discussion. 
The book before us gives an elementary but 
generally clear and skilful exposition of the present 
aspects of the evolutionary problem. It is the work 
of one whose sympathies are confessedly Mendelian 
and mutationist, but who shows a real desire to do 
justice to the views of opponents. Mr. Lock’s point 
of view is far removed from that of certain half- 
