I 



376 



NA TURE 



[I'EBKUARY 16, 1905 



tions, or of hard muscular labour ; there are also the unex- 

 |)l;iined deteriorations of age. His personal investigations 

 ii.nl been into the effects on the artiries of gradual increases 

 111 bliiod pressure. N'ormally, arterial pressures, as taken 

 in the arm, rise somewhat from childhood to age — say from 

 iSo-<)o mm. llg. to 140° or perhaps 150°. These upper limits 

 are not inconsistent with health at the age of three score, 

 though no doubt they signify some loss of mechanical 

 efliciency. .A demonstration was given by Dr. Dixon of 

 the difference in vascular efficiency under muscular effort 

 between a young and an elderly man. Into the effect of 

 certain poisons and infections on the arteries he could not 

 enter. Senile degenerations of the arteries are not essenti- 

 ally allied to rise of blood pressure, though in such subjects, 

 as in others, high pressures may arise, and n-.ust be, of cour.se, 

 the more dangerous. Still, senile arterial degeneration is 

 compatible with very long life, even if with diminution 

 of function, as the vessels silt up rather than burst. 



The lecturer's own observations, now extended over many 

 years, had been upon rise of pressure in middle life beyond, 

 ofli-n very far beyond, that which he had regarded as 

 normal for elderly persons. The reasons of this morbid 

 tendency cannot yet be given, but fortunately, by medicinal 

 and dietetic means, it can be abated, and in early stages 

 abolished. If permitted to persist, and it is not rarely 

 consistent with fair general health or but vague indis- 

 position, it slowly ruins the vascular system by over- 

 stretching it. It is in such persons thai the arteries mav 

 break, as in apoplexy, a catastrophe which, by timely pre- 

 cautions, can be prevented. The lecturer strongly urged 

 upon all persons of middle and advancing years to have 

 their arterial pressures tested by their physicians everv four 

 or five years, so that any disposition to excessive pressures 

 may be averted and the integrity of the arterial tree pre- 

 served. 



RADIATIOX PRESSLRE.' 

 A Hl'NDKED years ago, when the corpuscular theory 

 held almost universal sway, it would have been easier 

 to explain the pressure of light than it is to-day, when it is 

 certain that light is a form of wave-motion. The means at 

 the disposal of early experimenters were inadequate to detect 

 so small a quantity ; but if the eighteenth century philoso- 

 phers had been able to carry out the experiments of Lebedeff 

 and of Nichols and Hull, and had they further known of the 

 emission of corpuscles revealed to us bv the kathode 

 stream and by radio-active bodies, there can be little doubt 

 that \'oung and Fresnel would have had much greater difli- 

 cully in dethroning the corpuscular theory and" setting up 

 the wave theory in its place. The existence of pressure due 

 to waves, though held by Kuler, seems to have dropped out 

 of sight until Maxwell, in 1872, predicted its existence as a 

 consequence of his electromagnetic theory of light. The first 

 suggestion that it is a general property of waves is probablv 

 due 10 Mr. S. T. Preston, who in 187(1 pointed out the 

 analogy of the energy-carrying power of a beam of light 

 with the mechanical carriage by belting, ;md calculated the 

 pressure exerted on the surface of the sun bv the issuing 

 radiation. It seems possible that in all cases of energv 

 transfer, momentum, in the direction of transfer, is also 

 passed on and that there is, therefore, a back pressure on the 

 source. Though there is as yet no general and direct dv- 

 namical theorem accounting for radiation pressure. Prof. 

 I.armor has given a simple indirect mode of proving the 

 existence of the pressure which applies to all w.tves in which 

 the average energy density for a given amplitude is inversely 

 as the square of the wave-length. He has shown that when 

 a train of waves is incident normally on a perfectly reflecting 

 surface, the pressure on the surface is equal to F;(i + 2u/U), 

 where E/2 is the energy density just outside the reflector in 

 the incident train, U is the wave-velocity, and 11 the velocity 

 of the reflector, supposed small in comparison with U. In a 

 similar manner it can be shown (hat then- is a pressure on 

 the source, increased when the source is moving forward, 

 decreased when it is receding. It is essential, however, that 

 we should be able to move the reflecting surface without dis- 

 turbing the medium except by reflecting the waves. 



' Ad'trc's delivf-red before iht Physic.nl Pocicly on February 10 by Prof. 

 J. H. Poynting, F.R.S., president of ihc society. 



NO. 1842, VOL. 71] 



Though Larmor's proof is quite convincing, it is interest- 

 ing to realise the way in which the pressure is produced in 

 the different types of wave-motion. In the case of electro- 

 magnetic waves, Maxwell's original mode of treatment is 

 the simplest. A train of waves is regarded as a system of 

 electric and magnetic tubes transverse to the direction of 

 propagation, each kind pressing out sideways, that is, in the 

 direction of propagation. They press against tne source 

 from which they issue, against each other as they travel, and 

 against any surface on which they fall. In sound-waves 

 there is a node at the reflecting surface. If the variation of 

 pressure from the undisturbed value were exactly propor- 

 tional to the displacement of a parallel layer near the surface, 

 and if the displacement were exactly harmonic, then the 

 average pressure would be equal to the normal undisturbed 

 value. But consider a layer of air quite close to the surface. 

 If it moves up a distance, y, towards the surface, the pres- 

 sure is increased. If it moves an equal distance, y, away 

 from the surface, the pressure is decreased, but to a slightly 

 smaller extent. The excess of pressure during the compres- 

 sion half is greater than its defect during the extension half, 

 and the net result is an average excess of pressure on the 

 reflecting surface. Lord Kayleigh, using Boyle's law. has 

 shown that this average excess should be equal to the 

 average density of the energy just outside the reflecting sur- 

 face. In the case of transverse waves in an elastic solid, it 

 can be shown that there is a small pressure perpendicular to 

 the planes of shear, that is, in the direction of propagation, 

 and that this small pressure is just equal to the energy density 

 of the waves. The experimental verification of the pressure 

 of elastic solid waves has not yet been accomplished, but the 

 pressure due to sound-waves has been demonstrated by 

 .\ltberg, working in Lebedeff's laboratory at .Moscow, the 

 pressure obtained sometimes rising to as much as 0.24 dyne 

 per sq. cm. By means of a telephone manometer it was 

 found that through a large range the pressure exerted on a 

 surface was proportional to the intensity of the sound. 



Both theory and experiment justify the conclusion that 

 when a source is pouring out waves, it is pouring out with 

 them forward momentum which is manifested in the back 

 pressure against the source and in the forward pressure 

 when the waves reach an opposing surface, and which, in 

 the meanwhile, must be regarded as travelling with the 

 train. It was shown that ibis idea of momentum in a wave- 

 train enables us to see the nalure of the action of a beam of 

 light on a surface where it is reflected, absorbed, or refracted 

 without any further appeal to the theory of the wave-motion 

 of which we suppose the light to consist. In the case of total 

 reflection there is a normal force upon the surface, in the 

 case of total absorption [here is a force normal to the surface 

 and a tangential force parallel to the surface : while in the 

 case of total refraction there is a normal force which may be 

 regarded as a pull upon ihe surface or a pressure from 

 within. In any real refraction there will be reflection as well, 

 but with unpolarised light, in the case of glass, a 

 calculation shows that the refraction-pull is always 

 greater than the reflection-push, even at grazing incidence. 

 .\n experiment, made by the president in conjunction with 

 Dr. Barlow, was described to serve as an illustration of the 

 idea of a beam of light being regarded as a stream of 

 momentum. .V rectangular block of glass was suspended by 

 a quartz fibre so that the long axis of the block was hori- 

 zontal. It was hung in ;in exhausted case with glass win- 

 dows, and a horizontal beam of light was directed on to one 

 end of the block so that it entered centrally and emerged 

 centrally from the other end after two internal reflections. 

 Thus a stream of momentum was shifted parallel to itself, or 

 in this particular case a counter-clockwise couple acted on 

 the beam. By suitable means the clockwise couple on the 

 block, due to the pressures at the two internal reflections, 

 was distinctly observed and approximately measured. The 

 result obtained was of the same order as that deduced from 

 the measurement of the energy of the beam by means of a 

 blackened silver disc. 



The extreme minuteness of these light forces appears to 

 put them beyond consideration in terrestrial affairs, but in 

 the solar system, where they havi- freer play, and vast times 

 lo work in. their effects niay mount up into importance. On 

 the larger bodies the furce of the light of the sun is small 

 compari'd with the gravitational attraction, but as the ratio 

 of Ihe radiation pressure to the gravitation pull varies in- 



