Marcu 17, 1904 | 
WAT ORE 
463 
as the rays become harder. This result is in agreement with 
that given by Mr. Eve. I find also that in hydrogen, in 
which the ionisation is much less than in air, the ionisation 
increases relatively to that in air with the increase of hard- 
nes» of the rays. 
The experiments are not quite completed vet, but it is 
hoped to publish a full account of them shortly. 
R. K. McCiunc. 
Cavendish Laboratory, Cambridge, March 12. 
Polarisation in Roéntgen Rays. 
IN a paper on secondary radiation from gases subject to 
X-rays (Phil. Mag. [6] v., p. 685, 1903), I described experi- 
ments which led to the conclusion that this radiation is due 
to what may be called a scattering of the primary X-rays 
by the corpuscles (or electrons) constituting the molecules 
of the gas. More recently I have found that from light 
solids which emit a secondary radiation differing little from 
the primary, the energy of this radiation follows accurately 
the same law as was found for gases, so that the energy 
of secondary radiation from gases or light solids situated 
in a beam of Réntgen radiation of definite intensity is pro- 
portional merely to the quantity of matter through which 
the radiation passes. Experimental evidence points to a 
similar conclusion even when metals which emit a secondary 
radiation differing enormously from the primary «re used as 
radiators, though I have as yet only shown that the order 
of magnitude is the same in these cases. The conclusion 
as to the origin of this radiation is therefore equally applic- 
able to light solids, and probably to the heavier metals. 
As explained by Prof. J. J. Thomson (‘* Conduction of 
Electricity through Gases,’’ p. 268), on the hypothesis that 
Rontgen rays consist of a succession of electromagnetic 
pulses in the ether, each ion in the medium has its motion 
accelerated by the intense electric fields in these pulses, and 
consequently is the origin of a secondary radiation, which 
is most intense in the direction perpendicular to that of 
acceleration of the ion, and vanishes in the direction of that | 
acceleration. The direction of electric intensity at a point 
in a secondary pulse is perpendicular to the line joining this | 
point and the origin of the pulse, and is in the plane passing 
through the direction of acceleration of the ion. 
If, then, a secondary beam be studied, the direction of 
propagation of which is perpendicular to that of the primary, 
it will on this theory be plane polarised, the direction of 
electric intensity being parallel to the pulse front in the 
primary beam. 
If the primary beam be plane polarised, then the secondary 
radiation from the charged corpuscles or electrons has a 
maximum intensity in a direction perpendicular to that of 
electric displacement in the primary beam, and zero intensity 
in the direction of electric displacement. Prof. Wilberforce 
first suggested to me the idea of producing a plane polarised 
beam by a secondary radiator, and of testing the polarisation 
by a tertiary radiator. 
The secondary radiation from gases is, however, much 
too feeble to attempt the measurement of a tertiary. From 
solids I think it will be possible, and hope shortly to make 
experiments on this. 
It occurred to me, however, that as Rontgen radiation 
is produced in a bulb by a directed stream of electrons, there 
is probably at the antikathode a greater acceleration along 
the line of propagation of the kathode rays than in a direc- 
tion at right angles; consequently, if a beam of X-rays 
proceeding in a direction perpendicular to that of the 
kkathode stream be studied, it should show greater electric 
intensity parallel to the stream than in a direction at right 
angles. 
I therefore used such a beam as the primary radiation, 
and studied by means of an electroscope the intensity of 
secondary radiation proceeding from a sheet of paper in a 
en perpendicular to that of propagation of the primary 
cam. 
By turning the bulb round the axis of the primary beam 
studied, the intensity of this beam was not altered, but the 
intensity of the secondary beam was found to reach a 
maximum when the direction of the kathode stream was 
perpendicular to that of propagation of the secondary beam, 
and a minimum when these two were parallel. 
NO. 1794, VOL. 69] 
In one series of experiments the intensity of secondary 
radiation in a direction perpendicular to that of the primary 
| beam was compared with that in a direction making a 
small angle with the axis of the primary beam. ‘The latter, 
according to theory, should not vary with the position of 
the X-ray bulb. 
In a second series of experiments the intensity of secondary 
radiation in a direction perpendicular to the axis of the 
primary beam was compared with that of a small portion 
of the primary beam itself, when the bulb was in different 
positions. 
Lastly, the intensity of secondary radiation was measured 
in two directions perpendicular to that of propagation of the 
primary radiation and perpendicular to each other, while 
the intensity of the primary beam was measured by a third 
electroscope. 
The three methods gave similar results. 
In the last case, as the bulb was turned round as de-~ 
scribed, one secondary beam reached a maximum of in- 
tensity when that at right angles attained a minimum. 
When the bulb was turned through a right angle the 
former produced a minimum of ionisation while the latter 
produced a maximum. 
Two bulbs were used and the sizes of the apertures were 
varied, but the results were similar in all cases. 
The variation of intensity of the secondary beam amounted 
to about 15 per cent. of its value, but this, of course, does 
not represent the true difference, as beams of considerable 
cross section were studied, consequently secondary rays 
making a considerable angle with the normal to the direc- 
tion of propagation of the primary rays were admitted into 
the electroscope. 
The experiments are being continued. 
These results, however, are in agreement with the theory, 
and I think show conclusively that the X-radiation proceed- 
ing from a bulb is partially polarised. 
; Cuartes G. BARKLA. 
University of Liverpool, March to. 
The British Government and Marine Biology. 
IN a note in your issue of February 25 announcing the 
appointment of Mr. James Hornell, who, it is stated, acted 
as Prof. Herdman’s assistant during the Ceylon pearl 
oyster investigation, to the post of marine biologist 
to the Government of Ceylon and inspector of the pearl 
banks, it is said that ‘‘ the appointment is of interest as 
showing how in the recognition of science some of our 
colonies are in advance of the mother country. We have 
no ‘ marine biologist to the Government ’ here.”” 
Now although the latter statement may be verbally 
accurate, it appears to me to be misleading, and-one would 
seem to be justified in supposing that it has been made 
without full knowledge of the facts. 
At the present time the British Government is committed 
to an expenditure of 42,000l., to be spread over a period of 
three years, for the purpose of carrying out the British 
portion of the international fishery investigations, the pro- 
gramme of which, conceived in an eminently scientific spirit, 
has been drawn up by an international council comprising 
amongst its members some of the most distinguished of 
European marine biologists. In addition to this the 
Government has made for a number of years, and still con- 
tinues to make, a grant of 10001. a year to the Marine 
Biological Association, the declared object of which is the 
promotion of both scientific and economic marine biology ; 
public money has been spent on scientific fishery investi- 
gations in both Scotland and Ireland, and the Government 
has quite recently appointed Dr. A. T. Masterman, a well 
known and capable marine biologist, to the post of inspector 
of fisheries. 
To decline to acknowledge what is already being done is 
surely not the way to obtain increased support for scientific 
investigations in the future. E. J. ALLEN. 
Marine Laboratory, Plymouth. 
Tue brief statement contained in the note was quite 
correct, and although it might be expanded and illustrated, 
it needs no qualification. We were well aware of all the 
facts stated by Dr. Allen. 
