May 27, 1915| 
NATURE 
343 

the Ituri forest hunting this scarce and extraordinary 
animal, and searching for the hypothetical Elephas 
pumilio (‘Elephant nairn”’), accompanied each day by 
one of my friends, the little Bambuti pigmies, without 
whose co-operation and assistance I could have done 
very little indeed. Elephants were everywhere, but 
not a sign of any pigmy species could I discover in the 
Ituri. 
On several occasions I saw the olapi alive, some- 
times at very close quarters, but so ghost-like, wary, 
and elusive is it, so difficult to track, even for the 
little ‘‘animal men,’ and so difficult is it to see in 
the prevailing gloom, that I only succeeded in shoot- 
ing two. A third one (No. 531) of the series was shot 
by Commandant Hedmark, an officer in the Congo 
service, who spent three days shooting with me at 
one of my forest camps, and came upon the animal 
quite by accident. 
A fourth (No. 717) was killed, late one afternoon, 
in mistake for a buffalo, by Mr. A. E. H. Reid, a 
prospector, also in the Congo service, and given to 
me. This specimen is of considerable interest, being 
an old male with well-developed horns some 4 in. in 
length, and skin-covered except at the tip, where, 
instead of a tuft of hair, the bony horn-core is bare, 
somewhat cup-like in shape, and of modified, white, 
polished, very hard, compact bone. The remaining 
three skins were procured from natives and are very 
incomplete. 
Hitherto there has been considerable confusion as 
to the sex to which the comparatively few skulls 
handled by naturalists belonged, and mainly for this 
reason there has been a doubt as to whether both 
sexes or only the male develops horns. ‘There is no 
question'as to the sex of four of my specimens, three 
males and one female. Two of the males have horns, 
but the third, being very immature, has not yet 
developed them. The big female, although an old 
animal, as shown by the condition of the teeth, has 
no horns or any signs of developing ossicones. This 
being so, I think I am justified in saying that my 
specimens go far towards proving that only the male 
okapi carries horns. 
They also help to prove that the female olsapi, 
unlike most other animals, attains a greater height 
and bulk than the male, a peculiarity suspected by 
a writer in Country Life (October 25, 1913) as the 
result of measurements of mounted specimens. 
Giraffe-like as the animal seems, and is, it is only 
when extremely young that the backward slope of the 
back is very noticeable. It does not feed on any 
species of water-plant so far as I know. In fact, it 
seldom frequents low situations near the water. Its 
food consists of leaves of the undergrowth and young 
saplings, and in feeding it reaches to a considerable 
height, pulling down leafy twigs with its long pre- 
hensile tongue. It does not, I think, feed at night, 
but in the early evenings and the mornings until at 
least as late as nine or ten o’clock. It is not a jungle- 
loving animal at all, but prefers the higher and drier 
parts of the forest, where the trees are big and the 
undergrowth comparatively scanty. It has no skulking 
bongo-like habits, but is never seen in the open. 
When going away at speed its neck is held straight in 
front, and it will jump obstacles rather than go be- 
neath them, like the bongo and the little red buffalo. 
Everything points to the okapi being the progenitor 
of the giraffe, or at least there seems little doubt that 
both are from the same stock. The okapi certainly 
does not appear to me to have any affinities with the 
antelopes. CUTHBERT CHRISTY. 
Royal Societies Club, St. James’s Street, S.W., 
Mav 24. 
NO. 2378, VOL. 95] 


A Further Extension of the Spectrum. 
In Nature of May 7, 1914 (vol. xciii., p. 241), I 
stated that I had extended the ultra-violet limit of the 
spectrum to the neighbourhood of wave-length goo 
Angstrom units. 
I have now succeeded in carrying my observations 
to wave-length 600. This result is chiefly due to the 
use of helium of considerable purity in my spectro- 
scope and discharge tube. The apparatus is the 
same grating vacuum instrument which I have em- 
ployed for several years, but perfected in such a way 
as to make it much more nearly air-tight than ever 
before. The spectrum which is obtained with a dis- 
ruptive discharge in helium contains, between wave- 
length 1250 and 600, upwards of fifteen lines, some 
of them of some strength. 
My work with hydrogen confirms the existence of 
the series predicted by Ritz with members at 1216, 
1026, and 972. But, owing to the great difficulty of 
obtaining the gas content of my spectroscope abso- 
lutely free from impurities, I am not even yet able 
to identify positively the source of certain strong lines 
which occupy the positions demanded by the analogue 
of the Pickering series, and occur both when 
hydrogen and when helium are employed. 
THeopore Lyman. 
The Jefferson Laboratory, Harvard University, 
May 11. 

The Distribution of the Electrons in Atoms. 
Tue spectra which are obtained by the diffraction 
of X-rays by crystals are characteristic both of the 
substance which emits the X-rays and of the crystal 
which acts as the grating. If the lines of an accu- 
rately ruled plane grating are small in width com- 
pared with their distance apart, the intensities of the 
different orders of spectra are nearly the same. If, 
however, the lines have a width comparable with the 
grating constant, the intensities of the higher orders 
rapidly diminish. When a crystal diffracts a beam of 
X-rays, the different layers of atoms correspond to the 
lines of the ordinary transmission grating, so that 
the relative intensity of the higher orders of spectra 
will depend upon the ratio of the effective diameter 
of the atoms in scattering the X-rays to the distance 
between the successive layers of atoms. 
There are good reasons for believing that it is the 
electrons in atoms which scatter the X-rays. On this 
assumption it may be shown that if the density of the 
space distribution of the electrons in each layer of 
atoms is some function f(z) of s, where the < axis is 
taken normal to the reflecting planes, the ratio of the 
amplitude of the nth.order spectrum to the amplitude 
it would have if all the electrons were in the same 
plane is :— 
ig f(s) cos(B+2mn2/d )dz 
a 


P . 
Po cos (2 + mn) [" [(2)dz 
Oars 
where b—a=d is the grating space, and f is the 
phase angle of the reflected ray. If it is possible to 
find some function f(z) which will lead to the values 
of P/P, as determined experimentally, an indication 
will be obtained of the distribution of the electrons 
in the atoms. 
W. H. Bragg has published experimental results 
(Phil. Mag., vol. xxvii., p. 895, 1914) showing the 
rate of variation of the intensity with the order when 
X-rays are reflected from rock-salt. It can be shown 
from his data that the intensities of the different 
