430 
NATURE 
[Mancu 4, 1897 
where C is also an independent and normal variable in respect 
to its intrinsic qualities, but subjected to the condition that the 
same value of C is to be used as the divisor of éo¢h members 
of the same couplet of Aand B. In short, that the couplets 
shall always be of the form A/C,, B/C,, and never that of 
A/Cn, B/Cn- 
Physical Society, February 26.—Mr. Shelford Bidwell, 
President, in the chair—Mr. J. H. Vincent read a paper on the 
photography of ripples. If mercury is used as the medium, all 
waves less than 1°3 cm. long come under Lord Kelvin’s defini- 
tion of a ripple; that is to say, they are waves whose lengths 
are less than such as are propagated with minimum velocity. 
Vibrations in mercury of about 200 per second and upwards 
generate waves whose propagation is controlled almost entirely 
by surface tension, and these waves are therefore classed as 
‘*capillary ripples.” Their speed of propagation is of the order 
of about one foot per second. They are invisible owing to their 
high frequency, and not in consequence of the velocity of their 
propagation. It is usual to examine them by some stroboscopic 
method. Mr. Vincent obtains photographs of the disturbed mer- 
cury surface by the sudden illumination of an electric spark. The 
. spark is about half a centimetre in length, and it lasts about one 
two-hundred-thousandth part of a second. Its brightness is 
increased by an auxiliary spark-gap. The optical arrangement 
consists of two lenses, one in the path of the incident light, and 
another to converge the reflected light from the mercury surface 
into a photographic camera. Ripples are set up in the mercury 
by a stylus attached to a tuning-fork. For this purpose 
it is generally sufficient to give a slight blow to the prongs; but 
when continuous vibration is required, the tuning-fork can be 
connected bya thread to an electrically-driven fork, as suggested 
by Mr. Watson. The first photograph shows a series of circular 
waves, set up by a single stylus attached to a fork vibrating 180 
times a second. Fixed points at known distances, just above 
the mercury surface, enable the wave lengths to be deduced 
rom the photographs; and, as the frequency is known, the 
fsurface-tension may be easily calculated. In a second photo- 
graph, two styluses are attached to the same prong. Dark lines 
are seen to radiate from the region between the centres of oscil- 
lation ; these are the lines of minimum disturbance—hyperbolas, 
of which the centres of disturbance are the foci. This photo- 
graph illustrates ‘‘ interference ” similarly to the optical methods 
of Young and Fresnel. A third photograph shows the forma- 
tion of elliptical curves of disturbance, being the loci of the 
intersection of two series of circles, corresponding one to each 
of two centres of vibration. Unlike the system of hyperbolas, 
these ellipses are not at rest, but travel outward from the sources. 
In order to render these ellipses stationary it would be necessary 
to change one of the sources into a szzk. towards which the 
circular waves might converge; the photograph would then 
correspond to the optical device of M. Meslin, who obtains 
interference fringes by means of a screen placed between two 
point centres, one a source, and the other a szwk, The 
phenomena of interference and diffraction are well shown ina 
photograph of a point source and a reflecting line. The reflector 
here is one side of a triangular piece of microscope cover-glass. 
The interference lines are due to the mutual action of incident 
and reflected rays; they are analogous to Lloyd’s single-mirror 
fringes. Other photographs exhibit analogues of ‘* spherical 
aberration” and ‘‘forced vibration.” Mr. Vincent acknow- 
ledged his indebtedness to Mr. Boys for the recommendation 
of attempting the photography of capillary ripples. Mr. 
Boys congratulated the ‘author upon the way in which the 
experimental difficulties had been overcome. The results would 
bear a good deal of close examination, and they would be 
found to present analogues of the greatest service in demon- 
strating the phenomena of acoustics and optics. Such photo- 
graphs were far better than geometrical pictures drawn by 
instruments. For example, in the photograph illustrating the 
regions of minimum disturbance by lines radiating from a two- 
point source, it was easy to make out the positions where the 
two series of waves were half a period behind one another. 
The crests and troughs appeared as a set of dark and light con- 
centric alternating circles, broken up into short arcs by radiating 
lines, the loci of minimum disturbance ; all the crests on one 
side of any particular radiating line were seen to correspond to 
troughs on the other side, so that the field of disturbance was 
mapped out as in acoustics. One set of phenomena yet awaited 
illustration by this photographic method, and that was ‘* dif- 
fraction” from a grating. It might be possible to use as an 
NO. 1427, VOL. 55] 
exciter a comb with chisel-shaped points. He did not think it 
would be possible to go quite so far as to reproduce analogues 
of spectral analysis. Since wave-length varies with surlace- 
tension it was possible to vary the wave-length by dropping a 
little ox-gall or soap solution upon the mercury surface. Mr. 
Blakesley asked why no reflections occurred from the sides of the 
mercury retainer. Mr. Boys said the waves were lost at the 
edges of the meniscus. The mercury was kept in position by an 
annular ring of thin glass. Mr. Appleyard suggested that the 
analogue of refraction might be obtained by an alteration of the 
surface-tension over a small area, by amalgamation or other 
means. Mr. Vincent thought this could be done, but that it 
would be very difficult. The President proposed a vote of 
thanks to the author.—Mr Elder then read a paper, by Mr. 
Beckit Burnie, on the thermo-electric properties of some liquid 
metals. The investigation was made with a view to determining 
the effect of melting upon the thermo-electric properties of 
certain metals. The metal to be tested is contained in a W- 
shaped glass tube, of which one limb can be cooled and the 
other heated. Thus one limb can contain molten, and the 
other solid, metal. Copper-wires are plunged, one into each 
limb, and through these connection is made with a galvanometer. 
The thermal-junctions, therefore, are copper-hot metal, and 
copper-cold metal. Thetemperature is deduced from a separate 
thermal-couple, calibrated by a mercurial thermometer. Curves 
are drawn coordinating temperature and electro-motive force. 
It is found that their slope depends upon the rate of cooling or 
heating of the metals ; this is particularly the case with bismuth. 
The effect is attributed to the variation in crystalline structure ot 
the metal under test, at different rates of solidification. With 
tin the change is less marked, and with lead it is unnoticeable. 
At or about the melting points, there is considerable change of 
slope in the curves. Here, again, the effect is smallest for lead ; 
somewhat greater with tin; and remarkably large with bismuth; 
the latter changing from an exceedingly active thermo-electric 
metal to one resembling lead. A great change occurs also with 
mercury at the melting point, indicating a difference in the 
Peltier effect between solid and molten metals.—A vote of thanks 
to Mr. Beckit Burnie was proposed by the President, and the 
meeting adjourned until March 12. 
Linnean Society, February 18.—Dr. D. H. Scott, F.R.S..,. 
Vice-President, in the chair.—Mr. J. E. Harting exhibited 
under a glass case the nest of a wren built of moss in the dried 
body of a rook which had been hung up as a scare-crow in 
Gloucestershire. Similar instances of the kind had been recorded 
(Zssex Nat. ii. 205 and iii. 25). He called to mind the nest of 
a swallow in the dead body of an owl mentioned by Gilbert 
White, and referred to other cases which had been collected by 
a former President of the Society (Bishop Stanley, ‘* Iist. 
Birds”). For instances of nests of the hoopoe placed in the 
desiccated bodies of unburied men, he referred to the experience 
of Pallas in Russia and of Swinhoe in China.—On behalf of Mr. 
D. T. Gwynne Vaughan, Dr. D. H. Scott gave the substance 
of a paper on the morphology and anatomy of certain 
Nympheacee.—Mr. J. H. Burrage read a paper on the adhesive 
dises of Ervcz//a spicata, Moq. 
Entomological Society, February 17.—Mr. R. McLach- 
lan, F.R.S., Vice-President and Treasurer, in the chair.— 
Messrs. Champion and Jacoby exhibited the collection of 
Phytophagous Coleoptera made by Mr. H. H. Smithin Grenada 
and the Grenadines for the West India Exploration Committee 
of the Royal Society.—Mr. F. C. Adams exhibited rare Dip- 
tera taken in the New Forest during the preceding year, and 
including Cadlécera enea and Nephrocerus favicornts.—Mr. M 
Burr showed an example of an undetermined species of locust 
taken in the Post Office at Bedford Street, Strand, and six new 
species of Acrydiidce of different genera.—The Secretary ex- 
hibited a Cicada larva from which a fungus, probably Cordyceps 
sobolifera, was growing, which had been sent to the Society 
from Venezuela, with an inquiry as to its real nature.—The 
Rev. Dr Walker showed a series of Coleoptera, Hymenoptera,,. 
and Diptera, collected in the Orkney Islands during the previous 
season.—Mr Tutt exhibited bred examples of the extreme radiate 
variety of Spzlosoma dubrecepeda. This variety occurred natur- 
ally in Heligoland, and its existence in Great Britain was 
probably attributable to accidental importation.—Mr. Jacoby 
and Mr. Champion communicated a list of the Phytophagous 
Coleoptera obtained by Mr. H. H. Smith in St. Vincent, 
Grenada, and the Grenadines, with descriptions of new species. 
