382 
NATURE 
[Frepruary 18, 1897 
of these gases, neither has helium been detected. This latter 
substance seems to be confined to minerals which contain the 
heavy metals, such as uranium and thorium, and at present it 
has not been found in any simple silicate. 
Physical Society, February 12.—Special General Meeting. 
—The chair was taken by Captain Abney, who, as retiring 
President, referred to some of the changes which had occurred 
in the Society during the past year. The annual subscription 
had been raised, but a satisfactory number of new Fellows had 
been enrolled. The Society had lost two by death. A good 
deal of work had been done in the direction suggested by the 
discoveries of Rontgen.—The Treasurer, Dr. Atkinson, then 
presented his report and balance-sheet for the year 1896. There 
was evidence of improvement in the financial position, but there 
was still a deficiency to be met. Profits from sales of publica- 
tions had been small ; it was desirable to reduce the price of the 
volumes of ‘‘ Collected Papers of Joule and Wheatstone,” and 
to call the attention of physicists to these valuable records of 
classical work. Mr. Walker suggested that physical labora- 
tories, especially those in London, should be visited by Fellows 
of the Society, with a view to comparing notes as to the con- 
struction of apparatus ; professors of colleges and other institu- 
tions should be invited to appoint visiting days for this purpose. — 
Votes of thanks were passed to the retiring President, Council, 
and Officers, and also to the Council of the Chemical Society 
for the use of their rooms at Burlington House.—In replying, 
Captain Abney said that the coming year would probably bring 
about further improvements in the system of abstracting and 
indexing, by co-operation withother Societiesat home and abroad. 
He then read the list of Council and Officers for the year 1897-8. 
President, Shelford Bidwell, F.R.S. ; Vice-Presidents who have 
filled the office of President: Dr. Gladstone, Prof. G. C. Foster, 
Prof. Adams, The Lord Kelvin, Prof. Clifton, Prof. Reinold, 
Prof. Ayrton, Prof. Fitzgerald, Prof. Riicker, Captain Abney, 
Vice-Presidents, Major-General E. R. Festing, L. Fletcher, 
Prof. Perry, G. Johnstone Stoney. Secretaries: T. H. 
Blakesley, H. M. Elder. Foreign Secretary (new office), Prof. 
S. P. Thompson. Treasurer, Dr. Atkinson. Librarian, C. 
Vernon Boys. Other members of Council: Walter Baily, L. 
Clark, A. H. Fison, Prof. Fleming, R. T. Glazebrook, Prof. A. 
Gray, G. Griffith, Prof. Minchin, Prof. Ramsay, J. Walker. 
The newly-elected President, Mr. Shelford Bidwell, then took 
the chair, and an ordinary meeting was held. Mr. Blakesley 
read a paper by Mr. H. H. Hoffert, ‘On the use of very small 
mirrors with paraffin lamp and scale.” For the mirrors of 
reflecting instruments the author prefers small rectangular strips 
of microscope cover-glass, chosen thin and plane. These are 
first silvered and then cut to shape bya splinter of diamond 
embedded in wax. They are about $8 mm. long, by 1°5 mm. 
broad, and are suspended so that their longest sides are vertical. 
Rectangular mirrors suspended in this way are lighter, and have 
less inertia than round mirrors of equal aperture. A paraffin- 
lamp flame placed edgewise to the mirror gives sufficient il- 
lumination. The image of the flame is focussed on the mirror by 
a lens midway between them, it is a right, vertical line, and 
thus conforms to the shape of the mirror. A scale is fixed upona 
screen between the lens and lamp ; and the screen has a circular 
aperture just below the centre of the scale, provided with a 
vertical cross-wire. The relative position of screen and lens is 
adjusted so that an image of the wire is formed upon the scale 
after reflection at the mirror. Mr. Boys said he had frequently 
used small mirrors constructed as described by the author, and 
he could not see what was new in the method, except that a 
paraffin lamp had been found sufficiently bright for the purpose. 
It is desirable to diminish jnertia by choosing extremely thin 
glass. Microscope cover-glasses are generally supplied in 
squares or discs very fairly equal in size; if they are dealt out 
on a table like a pack of cards, their relative thickness can be 
judged by the note produced as they fall. Flatness can be 
estimated nearly enough by balancing them one by one upon 
the knuckle nearly level with the eye, and observing the reflec- 
tion of an illuminated straight edge, such as a window bar. All 
rejected glasses should be broken. The good ones can be 
further examined by a telescope and artificial star. A common 
** writing ” diamond is best for cutting the thin plates. Special 
care must be taken not to distort the mirror in fixing to the 
suspended system. If liquid shellac is used in the attachment, 
distortion will certainly occur, at any rate if it is applied through- 
out the whole length of the mirror. The best way is to make the 
NO. 1425, VOL. 55] 
attachment at a mere point, near the top of the mirror; using a 
speck of shellac as viscous as possible, and heating, if necessary, 
by radiation, not by conduction. Mr. Boys thought that a re- 
flecting prism near the mirror might be used in certain cases where 
a paraffin lamp with its inevitable vertical flame was required 
for horizontal projections. For general purposes, Mr. Boys 
prefers some such arrangement as the following: If the source 
of light is a point, a lens is employed, forming an image of 
the source upon the mirror. (If the source of light is a sur- 
face, this lens .is evidently superfluous.) The cross-wire is 
stretched near to the lens on the side towards the mirror. It is 
now necessary to focus the cross-wire upon the scale, and this is 
best done by a plano-convex lens fixed as near as possible to the 
mirror, with its plane face towardsthe mirror. The light passes 
twice through this lens. As it may be necessary to change the 
plano-convex lens from time to time, according to the distance 
of the scale, Mr. Boys attaches it with a little vaseline to a strip 
of plate glass in front of the instrument. One advantage of 
such an optical system is that it allows the instrument to be set 
up in the same position, with respect to the scale, at all times. 
Dr. Thompson pointed out that Mr. Hoffert had obtained his 
results using only ove lens, by properly chodsing the position of 
the cross-wire.—A vote of thanks was given to the author, and 
the meeting adjourned until February 26. 
Entomological Society, February 3.—Mr. Roland Trimen, 
F.R.S., President, in the chair.—Mr. F. Bates, Mr. D. D’A. 
Wright, and Mrs. E. Brightwen were elected Fellows of the 
Society.—Mr. Champion exhibited an extensive series of 
Coleoptera collected by Mr. R. W. Lloyd and himself in the 
Austrian Tyrol, and containing about 450 species, including 35 
of Longicornia, and about 20 of Otiorrhynchus. He also ex- 
hibited about 85 species of Coleoptera from Cintra, Portugal, 
collected by Colonel Yerbury, the most interesting of these 
being Carabus lusttanicus, F.; also two specimens of the rare 
Zeugophora flavicollis, Marsh., from Colchester. Mr. Tutt 
showed, for Mr. W. H. B. Fletcher, typical Zygaeua 
ochsenheimert, Zell., from Piedmont, and hybrids between 
a female of that species and Z. filépendude. The progeny 
was fertile zzter se, the males closely approaching Z. 
ochsenhetmeri, the females Z. fidégendu/e in character. He 
also exhibited, for Mr. J. B. Hodgkinson, a number of obscure 
British Microlepidoptera, some of which had been described as 
new species. The determinations were criticised by Lord 
Walsingham, Mr. Bower and Mr. Barrett, and the former 
speaker strongly deprecated the practice of positively 
recognising or describing obscure species from single or 
worn specimens, particularly when JPritish—Mr. Barrett 
showed specimens of the true Platyptzlia tesseradactyla, L. 
(= P. fischeri, Zell.) new to the United Kingdom, and taken in 
Co. Galway.—Mr. McLachlan exhibited cooked locusts 
(Schestocerca peregrina) sold in the market of Biskra, Algeria, 
and received from the Rey. A. E. Eaton, They were cooked 
whole, but the abdomen only was eaten. The President, Mr. 
Barrett, and Mr. Blandford made some remarks on the subject. — 
A paper was communicated by Dr. A. G. Butler, on ‘‘ Seasonal 
dimorphism in African butterflies,” which led to a long discussion, 
chiefly on the so-called ‘‘ dry-season”” and ‘‘ wet-season forms.” 
Mr. Merrifield stated that he had been unable experimentally to 
modify the colour and markings of Lepidoptera by variations in 
humidity. Mr. Tutt believed that Mr. Doherty had obtained 
“* wet season forms” of Oriental species by keeping the pupa in 
a moist atmosphere. ; 
EDINBURGH. 
Royal Society, February 1.—Lord Kelvin in the chair.— 
A paper by Dr. J. Clarence Webster, on the changes in the 
mucosa of the corpus uteri, and in the attached foetal mem- 
branes during pregnancy, was laid on the table. Prof. D’Arcy 
Thompson described a very simple logical machine.—A paper 
by Lord Kelvin, Dr. Beattie and Dr. Smolan, on the conduc- 
tive quality induced in air by Réntgen rays and by violet light, 
was read. (See page 343.)—Lord Kelvin read a paper on 
crystallisation according torule. For example, as the beginning 
of a crystal forming from molecules moving freely in a solution 
consider a cluster of 13 balls, one touched by 12 neighbours 
around it (model shown). This presents 8 triangular beds and 
6 square ones, on which a wandering molecule may lie down. 
Let a rule be that a wandering molecule takes the first square 
bed which comes in its way, but never takes a triangular bed if 
