AUGUST 13, 1914| 
NATURE 
611 
experience through the introduction of 
classes to the use of the microscope in petrology. 
The style is clear, and suflicient optical theory is 
introduced to add interest to determinative methods. 
The philosophic processes by which minerals are 
determined, affording as they do an insight into 
crystalline structure, are of far more value in class- 
work than actual specific identifications. The 
beautiful series of photographs of minerals as they 
appear in thin rock-slices is a very welcome 
feature, and gives special distinction to the book. 
As an “introduction to petrology ” the micro- 
scopic method often fails; this is seen, for in- 
stance, where the author (p. 103) regreis the 
difficulty of distinguishing the triclinic felspars 
present ina granite. Surely, in a coarse-grained 
rock, a fragment broken from the mineral wil 
supply material for other than optical tests. We 
do not know, again, how “microscopic investiga- 
tion should always enable the student to make the 
distinction” (p. 112) between foliation and bed- 
ding, seeing that the two so frequently coincide in 
schists. But Mr. Smith has given us the handiest 
and best illustrated introduction that we possess 
to an important aspect of rock-minerals, and has 
even included a coloured plate of Newton’s scale. 
If the student remembers that every rock-section 
has its parent rock, he may well place himself at 
an early stage of his work under the guidance of 
these lucid pages. eA. fC, 
Researches into Induced Cell-reproduction in 
Amoebae. By J. W. Cropper and A. H, Drew. 
Pp. 112+ plates. (The John Howard McFadden 
Researches, Vol. IV.) (London: John Murray, 
Ipine oe rice 5s. net. 
THE investigations described in this volume were 
undertaken with the view of supporting the theory 
of H. C. Ross, that cell-reproduction is brought 
about by certain chemical agents termed 
‘“auxetics,’”’ and that their effect is increased by 
the addition of other substances known as 
“kinetics.”” The authors claim to have _ con- 
firmed this theory, and to have shown that the 
mode of action of these substances is probably 
through the medium of enzymes. The presence of 
these agents in the environment is stated to pro- 
duce variations in the morphology of the organ- 
ism. Methods of cultivation of amoebe and their 
examination by the “jelly method,” are de- 
scribed. The encystment of an ameeba is stated 
to be due to the action of certain deleterious bac- 
terial products, and it is claimed that the subse- 
quent excystation is caused by other products 
which act on the cyst-wall from without, and are 
of the nature of ferments. A detailed account is 
given of the preparation of cultures of the amoeba 
with pure strains of different bacteria. The 
amoeba used in these researches was a species 
found by the authors living in a solution of sodium 
chloride (1 per cent.) and sodium citrate (3 per 
cent.) in the laboratory, and named by them 
Amoeba ostrea, A parasitic micrococcus, which 
was very deadly to the amceba, was also isolated 
and investigated. 
WO:- 2337, VOL. 932i] 
b 
large 
Song and Wings: a Posy of Bird Poems for 
Young and Old. By Isa J. Postgate. With a 
Preface by the Rev. Canon H. D. Rawnsley. 
Pp. xi+50. (London: Alexander Moring, Ltd., 
1914.) Price 2s. 6d. net: 
Miss PostGaTe’s pretty verse wiil serve a very 
useful purpose if, by arousing an interest in birds 
and bird-life, it assists in the arrest of the exter- 
mination of beautifuily plumaged birds for the 
gratification of the desire for barbaric adornment 
fostered by modern fashion. 
LETTERS TC THE EDITOR: 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
Asymmetric Images with X-Radiation. 
CONFIRMING our letter of July 16 on asymmetric 
haloes, we have found that the bands can be produced 
on one, two adjacent, three, or all sides of a square 
(lead), and when an obstacle is placed at or over one 
side the corresponding band only vanishes. These 
effects are independent of the incidence of the radia- 
tion. With V-shaped aperture, the apex resting on 
the plate, the bands on one side are sharply defined, 
and twice bent; on the other there are several, and 
they are diffuse and fainter. 
The double bend is due to the fact that within a 
short range two edges contribute as in light. 
With strips (steel), say, 4 cm. width, the white 
bands cross at various angles depending on the slope 
to the plate, but are afterwards dispersed at short 
distances from the edges they approach. 
The diametric asymmetry excludes polarisation, and 
since it increases directly with the distance from the 
axis through the “optimum” (i.e. 15° from the direc- 
tion at which light would be reflected) the rays must 
have some determination from the plane of the anti- 
kathode. In other words, they must be * polarised” 
in planes at successive angles to the direction of pro- 
pagation. The continuance of the bands within the 
shadow beyond the range of light diffraction and their 
varying asymmetry show that X-radiation is something 
more than light of very short wave-length, or other- 
wise light itself must possess unequal polarity in its 
structure. 
‘ 
I. G. RANKIN. 
W. F. D. CHAMBERS. 
go Gordon Road, Ealing. 
Unit of Acceleration. 
Ir is a little surprising to find in Dr. Shaw’s paper 
(December 16, 1913), “ . He (Mr. Whipple) points 
out that we have no special name for the unit of 
acceleration.” In Nature of June 25, 1914, Mr. 
Whipple proposed the name “‘leo.’’ So long ago as 
1909 Wiechert used the term “ gal”’ in the report for 
the Gottingen earthquake station for that unit, being 
the first syllable of Galileo, whence Mr. Whipple 
derives his ‘‘leo.”” Others, as well as myself, have 
used ‘‘ gal,” or rather “ milligal,” in analyses of earth- 
quakes. A milligal is approximately a millionth of g. 
Dyne is the unit of force, gal the unit of acceleration. 
Orto Kiorz. 
Dominion Observatory, Ottawa, July 18. 
