tl 
NATURE 
[May 7, 1885 
SOCIETIES AND ACADEMIES 
LONDON 
Physical Society, April 25.—Prof. Guthrie, President, in 
the chair.—The following papers were read :—On the theory of 
illumination in a fog, by Lord Rayleigh. The paper dealt with 
certain theoretical results based upon the assumption that the 
medium in which the fog was formed and the substance com- 
posing the fog itself were perfectly transparent. The effect of 
such a fog surrounding a source of radiation would be to 
diminish the radiation, and in the case of a supply of energy 
from without, as with the carbon filament of an incandescent 
lamp, the temperature of the source would be increased by the 
fog. A spherical envelope of such a fog surrounding the lamp, 
and sufficiently thick to be impervious, would act as a perfectly 
reflecting surface. A problem closely related to the above, and 
which is easily worked out, is that of light incident normally 
upon a pile of glass plates. If be the number of such plates, 
and p the fraction of incident light reflected by one plate, (7) 
the light reflected, and W(vz) that transmitted by a pile of 7 plates, 
we have— 
gim)_ tn) 
2mp I+(2m—-1)p I-p 
If the transmitted light be allowed to*fall upon another pile con- 
sisting of 2 plates, we have an infinite amount of reflection 
between the plates, and as the final result if 4A denotes the 
radiation in the original direction, and 4 that in the opposite, 
ye 2up+I-p 
2(+7)p 
2up 
2(m+2)p+ i 
If m and x are large, we have— 
{= 15 = 
Mt 
A , 
m+n 
which shows that by increasing x we can make the radiation 
between the plates as much as if the first pile did not exist what- 
ever the number of plates in it.—On a monochromatic telescope, 
by Lord Rayleigh. This is a modification of Maxwell’s colour- 
box. In this instrument, as is well known, light passes through 
a slit in the focus of a collimating lens ; it traverses in succession 
this lens, a prism, and another lens by which it is brought to a 
focus upon a plane surface in which is a movable slit, the eye 
being placed behind which receives light approximately mono- 
chromatic. If, in addition, a lens be placed just behind the first 
slit, so as to bring some distant object into focus at a convenient 
distance from the eye, this object will be seen by the light that 
would enter the eye in the simple colour-box. The author sug- 
gested the use of this instrument to compare lights of different 
colours, and hinted at the possibility of choosing some colour 
towards the middle of the spectrum at which light might be 
compared for practical purposes.—On the self-regulation of the 
compound dynamo, by Prof. A. W. Riicker. If ¢ represents 
the current or electromotive force in the external circuit of either 
form of compound dynamo, it is given by means of an equation 
of the form 
P Q 
A+x B+x 
where 4, &, P, and Q are quantities which are different in 
different cases, but are always independent of the external resist- 
ance, and x is the conductivity or the resistance of the external 
circuit, according as ¢ represents the E.M.F., or current. The 
constant 4 in all cases depends only on the resistance of the 
various parts of the machine. If u and mm are respectively the 
largest and smallest values of « between which self-regulation is 
aimed at, then «—m may be called the range of x. That value 
of « which corresponds to the resistance most frequently used 
may be called the wswa/ value of x and indicated by. The 
maximum efficiency 7 of the machine is connected with 4 and & 
by the relations 
A=t(1+7)/(1—7) if > be the external E.M.F. 
A=£(1+7)/(1+7) if » be the external current. 
It can easily be shown that the function @ has two critical values, 
and that the value of x, corresponding to one of these, is necess- 
arily negative, unless one of the inducing spirals is wound so as 
to diminish the magnetisation Warious cases are considered, 
corresponding to different relations among the magnitudes of the 
constants 4, B, P, and Q. The following indications of the 
method of treatment may suffice. If 4/B<1<,/P/,/0Q, gis 
positive for all positive values of .v, and the critical value of @ | 
occurs for a negative value of x, so that @ diminishes as « 
increases. Hence, if we write 
eeeOn 
Atm Bim " 
we must have 
BS es, SORE A 
At+mu Bt+p +9 
where g is a positive quantity which will be less as the self 
regulation is more perfect. These equations give 
_ > .n-m—g(B+m) 
SP UGS eS INE et A 
I+g Garena hal tere 
_ 9 Bho-m—glAtmn) 
~i+g (4-8) p—m) 
Now since 4 — & is negative, we must, if Pand Q are positive, 
have 
(B+p)B+m). 
g<(u—m)/(A +m), 
g<(u-—m)/(B+m). 
By similar methods inferior limits to g are found in other 
cases, and it is thus shown that for given values of « and m, 
the limit is lower as 4 is larger. It has, however, been proved 
above that if the maximum efficiency of the machine is high, 4 
will te large or small, according as it is taken from an expres- 
sion that gives the external E.M.F. or the external current. 
Hence it is more difficult to combine high efficiency with good 
self-regulation if an approximately constant external current is 
desired than if an approximately constant external E.M.F. is 
aimed at. The equations do not lead to any simple rules for the 
relations which should hold between the various parts ofcompound 
dynamos ; but if some of the constants are taken as given, the 
values which must be assigned to the others can be calculated if 
a given efficiency for the usual value of x and a given deviation 
from perfect self-regulation between given values of x are to be 
attained.—On the determination of the heat-capacity of a ther- 
mometer, by Mr. J. W. Clark. The method consists in the 
estimation of the masses of the mercury and glass of the 
thermometer by weighing the instrument in air and in water, 
and again in water when immersed to the extent usual in the 
thermal experiment. The specific gravity of the glass and 
mercury being known, the absolute masses immersed can be 
readily calculated, and consequently their thermal capacity.—A 
photometer which enabled a comparison to be made between 
the light of a lamp emitted at any angle anda standard was 
exhibited by Mr. Dibdin, and the action explained by Mr, 
Livingstone, who stated that the maximum amount of illumina- 
tion took place at an angle of 45°. a 
Geological Society, April 15.—Prof. T. G. Bonney, 
F.R.S., President, in the chair.—John Rudd Leeson, M.D., 
was elected a Fellow of the Society.—The following communi- 
cations were read :—A general section of the Pagshot strata from 
Aldershot to Wokingham, by the Rey. A. Irving, F.G.S. The 
author referred to earlier papers in the Geological Magazine, in 
which the green colouring-matter so common in the Middle and 
Lower Bagshot strata of the London Basin had been attributed 
to the presence of vegetable débris and the materials resulting 
from decomposition of vegetable matter. The marked difference 
in this respect between these strata and the higher members of 
the series furnishes a clue to the conditions under which they 
were respectively deposited, the former being delta- and lagoon- 
deposits, the latter the deposits of a marine estuary. This im- 
plies a transgressive overlap of the upper portions of the Bagshot 
series upon the London clay ; and the present paper was devoted 
to a consideration of the stratigraphical evidence of this overlap. 
Sections were described in detail at Aldershot, Farnborough, 
Yateley, Camberley, Wellington College and the neighbourhood, 
and from the last-named place to Wokingham. From these a 
general section was constructed to exact scale, both as to thick- 
ness of strata and altitudes, showing a relation of the Bagshot 
formation to the London clay which was inconsistent with the 
generally received idea of their conformability and at variance 
with the mapping of the district as executed by the Geological 
Survey. The importance of the Bagshot pebble-bed as a base- 
ment-line of the upper division of the Bagshot strata was shown, 
as was suggested by the author so long ago as 1880. The syn- 
clinal arrangement of the London clay was shown to have been 
produced defore the deposition of the Bagshot series, though a 
and a fortior?. 
