APRIL 1, 1897 | 
NALURE 
525 
SOCIETIES AND ACADEMIES. 
Lonpon. 
Royal Society, March 4.—‘‘ Luminosity and Photometry.’ 
By John Berry Haycraft, M.D., University College, Cardiff. 
The luminosity of the spectrum was determined by the method 
of the ‘‘ minimal effective stimulus,” the portions of the spectrum 
investigated being as a physical quantity reduced in amount 
until its effect on the visual apparatus was just apparent. The 
luminosity was also determined by the ‘‘ flickering’ method. 
A rotating semi-dise periodically cut off the spectral ray, and 
produced flickering, which flickering disappeared after a certain 
speed of rotation had been reached : the speed of this rotation 
was taken as a measure of the luminosity. The curves obtained 
by these methods agreed with each other and with curves ob- 
tained by methods of ‘‘inspection” used by Abney and Konig. 
The curves obtained with the dark adapted eye—the observer 
was kept in a dark room during and for an hour before 
the experiment—gave a maximum in the green in the case 
of the minimal effective stimulus. With the light adapted eye 
—the room was whitewashed and lit by gas—the yellow of the 
spectrum was the most luminous. With the flicker method the 
curve of a spectrum of low physical luminosity has a maximum 
in the green, the curve of a spectrum of high luminosity in the 
yellow. Purkinje’s phenomenon was also studied by the above 
methods, using coloured papers and a graduated gas-burner to 
vary the luminosity. The full paper will shortly appear in the 
Fournal of Physiology. 
Physical Society, March 26.—Mr. Shelford Bidwell, 
President, in the chair. At the invitation of Dr. S. P. Thomp- 
son, the meeting was held at the Technical College, Leonard 
Street, Finsbury. Mr. Rollo Appleyard read a paper on 
liquid coherers and mobile conductors, and showed the follow- 
ing experiments: (1) A glass tube, containing mercury and 
paraffin-oil, is shaken up until the mercury divides into small 
spheroids. ‘The resistance of the chain of spheroids under these 
conditions is several megohms. Coherence can be brought 
about by a direct current, a spark, or by a Hertz oscillator. 
The coherence is visible, the spheroids forming into large glo- 
bules. At the same time, the resistance falls to a fraction of an 
ohm. (2) An unstable emulsion is formed by shaking water 
and paraffin oil together, in a glass tube; called by the author a 
‘*rain” tube. The oil may be coloured with alkanet root. By 
_ electrification, the water suspended in the oil is suddenly pre- 
cipitated in a shower through the oil, precisely as rain is pre- 
cipitated in the air, after thunder. (3) A mixture of paraffin oil 
and water is poured into a photographic dish, just covering the 
bottom ; and a little mercury is poured in. Any two separate 
globules of mercury in the dish are then connected by wires to 
a battery of about 200 volts, through a reversing-key. A 
momentary tap of the key causes instantaneous deformation of 
the mercury, especially of the globule connected to the negative 
pole. If the current is kept on, the negative globule sends 
forth a long tentacle of mercury across the dish to the positive 
globule. The tentacle may break into spheroids. Intermediate 
globules send forth ‘‘ fingers” towards the positive terminal 
globule ; and, by continued application of the current, the 
** fingers” link intermediate globules ; illustrating the nature of 
liquid coherence. By using the current-reverser asa telegraphic 
transmitting-key, the motions, to right or left, of the ‘‘ finger” 
of any stray globule may be interpreted to form the letters of 
the Morse code. Bya succession of taps of the key in one 
direction or the other, a globule can be made to “‘ caterpillar ” 
along the dish. Prof. Ramsay said he had once attempted to 
facilitate churning by the application of 8 or 9 volts to some 
milk. He thought the cream came a little faster, but it turned 
sour very quickly. Prof. Fitzgerald thought that the effects ob- 
served in experiment (3) were the result of current, and not of 
electro-static changes, and he would like to know the value 
of the actual current used. There was no doubt that 
the motions were due to variations in capillarity. Mr. 
Shelford Bidwell asked how the mercury was formed into 
spheroids in the tube in experiment (1). Mr. Appleyard, in 
replying to Prof. Fitzgerald, said it was not easy to define the 
circuit, as the terminal-globules were rather capricious, but he 
would try and measure the current in some particular case. The 
mercury-tube in experiment (1) was shaken in a horizontal 
plane ; the operation took about ten minutes. Equal volumes 
of mercury and oil was a good proportion. One-quarter of the 
NO. 1431, VOL. 55 | 
length of the tube should be left as an air-space.—Prof. Dalby 
then exhibited five pieces of apparatus: (1) a kinematic slide, 
(2) an inertia apparatus with trifilar suspension, (3) a Wilber- 
force spring, (4) an Ewing's reading-telescope, (5) a kinematic 
Hook-gauge. Models (1), (2), (4), and (5) illustrated the 
various degrees of freedom of bodies restrained at different 
numbers of points. It was shown with (3), that in extending a 
spiral spring there results a certain amount of twisting. Ifa 
mass is hung at the lower end of the spiral in such a way that, 
when suddenly released after extension of the spring, the time 
of oscillation of the mass in the horizontal plane (rotation) is the 
same as the time of vertical oscillation, then the tendency to 
twist results in a change of energy which alternates between the 
rotary and linear forms. Mr. Boys drew attention to the con- 
ditions of restraint, and suggested a criterion for determining 
whether a piece of mechanism was designed for minimum strain 
on the structure: a thin wedge slipped under any one point of 
contact should not disturb the other points of restraint. Prof. 
Fitzgerald pointed out the effect of symmetry upon the motion 
of the spring of (5). The spiral happened to be an unsym- 
metrical form; the change of phase from vertical to rotary 
oscillation was therefore rapid. In the case of the vibra- 
tion of a symmetrical stretched cord the change of phase 
would be very slow.—Dr. Thompson exhibited two 
kinematic models depending upon the principle that 
any simple harmonic motion may be considered as 
the resultant of two oppositely-directed motions. The first 
illustrates the synthesis of two opposite circular motions of equal 
period and amplitude to form a straight-line motion ; the second 
shows the combination of two simple harmonic motions of equal 
period and amplitude in any difference of phase, to form a 
circular motion. In each case the motion is communicated toa 
stylus by a link-gear, operated by two wheels rotating in opposite 
directions. In the first apparatus, the wheels are pivoted about 
their centres, and the link-gear is pinned to one point on the flat 
surface of each wheel, near the circumference ; in the second 
apparatus, the wheels rotate as eccentrics at 180° to one another, 
and the motion to the link-gear is communicated by thrust-rods, 
held by springs against the peripheries of the corresponding 
wheels. Dr. Thompson further exhibited a device for projecting, 
by lantern, the rotating magnet and copper disc, of Arago. The 
curious rotations and lateral movements of iron-filings, in a re 
volving magnetic field, were similarly projected on a screen. 
He also showed some experiments with a heat-indicating paint, 
made from a double iodide of copper and mercury, discovered 
twenty years ago by a German physicist. At ordinary tempera- 
tures the paint is red, but at 97° C. it turns black. If paper is 
covered with this substance, and then warmed at a stove, the 
change is effected in a few seconds. Various designs can be 
wrought upon the back of the paper in dead-black or gold, so 
that when warmed they appear in red or black on the front, 
according to their respective absorptive powers. Or local cool- 
ing by the hand will yield a silhouette. If the paper is allowed 
to cool, the silhouette vanishes, but it appears again when the 
paper is reheated. It has thus a kind of thermal ‘‘ memory.” 
A yellow double iodide of silver and mercury is even more 
sensitive. It changes from yellow to dark red at 45° C. Lastly, 
Dr. Thompson exhibited a kinematic model of Hertz-wave 
transmission. A row of lead bullets is suspended from strings, 
so that the bullets hang clear of one another by about an inch, 
in aright line. The strings are meshed, and herein the model 
differs from the well-known wave-models used in acoustics. If 
the attempt is made to send an acoustic form of wave 
through the system, by giving an impulse to the first 
bullet in the plane of the other pendulums, it fails im- 
mediately, owing to the slackening of parts of the 
meshes. Thus, only ¢vavsverse vibrations can be transmitted. 
To illustrate the propagation of a Hertz-wave, a heavy pendu- 
lum, oscillating in a plane at right-angles to the line of bullets 
at one end, represents the Hertz ‘‘ oscillator.” A metal ring, 
mounted horizontally on a trifilar suspension, and properly 
“tuned,” represents, at the distant end, the Hertz ‘‘ resonator.” 
Waves, formed by the transverse vibrations of successive bullets, 
are then propagated from end toend. Prof. Fitzgerald said the 
model was specially interesting as illustrating the difference in 
velocities of propagation of a given wave, and’ of the energy 
corresponding to it. The model did not accurately compare 
with ether, because in ether the rate at which the energy is 
propagated is the same as that of the wave. The difference of 
the two rates, for any medium, depended upon the “‘ dispersion * 
