472 
WATURE 
[Marcu 18, 1897 
and photographs. Vice-President Ryerson and Mr. C. L. 
Hutchinson, on their trip around the world, procured and pre- 
sented to the Museum a large and unique amount of material, 
including Etruscan and Stone Age remains from Italy, Roman 
terra-cottas, metal and stone work from the Indies, and butter- 
flies from the Himalayas. Mr. Owen F. Aldis invited Mr. O. 
P. Hay, Assistant Curator of Ichthyology, to accompany him 
on an excursion to the waters of Southern Florida. Nearly one 
hundred fine specimens were thus obtained. By sending col- 
lectors to all parts of the world in this way, the Field Columbian 
Museum gives evidence of very great activity. The specimens 
obtained by its officers will not only serve to enrich the Museum 
directly, but a large number of them can do so indirectly by 
exchange. 
THE additions to the Zoological Society’s Gardens during the 
past week include two Secretary Vultures (Sevfentartus reple- 
vorus) from South Africa, presented by Mr. P. Myburgh ; two 
Sacred Ibises (Zé2s ethéopicus) from South Africa, presented by 
Mr. Almeda; a Herring Gull (Larus argentatus), a Lesser 
Black-backed Gull (Larws fuscus) from Nova Zemlya, pre- 
sented by Mr. C. L. Rothera; a Rose-crested Cockatoo 
(Cacatua moluccensis) from Moluccas, presented by Mrs. Ander- 
son ; two Crested Porcupines (Hystorzx crzstata), a Griffon Vul- 
ture (Gyps fulous) from North Africa, presented by Mr. R. S. 
Hunter; four Common Rat-Kangaroos (Poterous tridactylus,2 8, 
22), seventeen Lesueur’s Water Lizards (Phys¢gnathus lesueur?) 
from Australia, deposited; two Parrakeets (Psephotus 
chrysopterygius) from Australia, four Brent Geese (Bernzcla 
brenta, 28,2), European ; a Bengalese Cat (els bengalensts) 
from the East Indies; four Red-crested Pochards (/wdigula 
rupina, 2,29) from India, three Mandarin Ducks (4x galerz- 
culata, @) from China; three Summer Ducks (.2x sponsas, ? ) 
from North America ; two Rosy-billed Ducks (Jetopzana pepo- 
saca, 6) from South America; a Japanese Teal (Qzerguedula 
formosa, 9 ) from North-east Asia ; five Chiloe Widgeon (J/areca 
stbilatrix, 3é,29) from Chili, a Spur-winged Goose (P/ectro- 
pterus gambensis, 4) trom West Africa, purchased. 
OUR ASTRONOMICAL COLUMN. 
CouDE MOUNTINGS FOR REFLECTING TELESCOPES.—For 
spectroscopic work the reflector is, without doubt, the most 
ideal form of telescope. Not only does the visual light-grasping 
power increase very rapidly the larger the aperture, but for the 
purposes of photography the same is also true. The refractor 
has, however, as yet the most convenient and comfortable 
arrangement for observation from the observer’s point of view, 
while with the reflector the observer is not so conveniently 
situated. That some kind of coudé arrangement can be adopted 
is, therefore, an important step in bringing these instruments 
more into use; for not only are reflectors cheap when 
compared with objectives, but their mountings and the 
accompanying housing are much less expensive. Prof. Wads- 
worth, in the February number of the Astrophysical Fournal 
(vol. v. No. 2), describes several ways in which the reflector may 
be coudé mounted, one of which was suggested by the late Mr. 
Cowper Ranyard, but was not completely worked out owing to 
his sudden death. Perhaps the two most promising arrange- 
ments are (I) when the reflector is of the Newtonian type, and 
the primary flat is placed at right angles to the axis of the tube 
reflecting the cone of rays back again on to two small mirrors, 
one placed justin front of the mirror, and the other in the polar 
axis ; and (2) when the reflector is of the Cassegrain type, and 
a single small additional mirror is necessary to reflect the rays 
directly down the polar axis. The latter appears, however, the 
more simple of the two, but the method of mounting seems 
somewhat too weak for mirrors of large size. Prof. Hale, in the 
same number of that journal, discusses the comparative value of 
refracting and reflecting telescopes for astrophysical investiga- 
tions, pointing out the superiority of the latter from many points 
of view, while Mr. Ritchey describes a new method of a support 
system for large specula. 
NO. £429, VOL. 55] 
ON APPARENT AND REAL DISELECTRIFI- 
CATION OF SOZID DIELECTRICS PRO- 
DUCED BY RONTGEN RAYS AND BY 
FLAME 
HE fact that air is made conductive by flame, by ultra- 
violet light, by Réntgen rays, and by the presence of bodies 
at a white heat has been shown experimentally by many experi- 
ments. We propose in this communication to give some results 
bearing on this conductivity of air, based chiefly on experiments 
of our own. 
We have examined more particularly the behaviour of paraffin 
and of glass. ; 
In our first experiments with paraffin we used a brass ball or 
about an inch diameter, connected to the insulated terminal 
of an electrometer by a thin copper wire soldered to the ball. 
The ball and the wire were both coated to the depth of about 
one-eighth of an inch with paraffin. The ball was then laid on 
a block of paraffin in a lead box with an aluminium window, 
both of which were in metallic connection with the case of the 
electrometer. By this means we avoided all inductive effects. 
The electrometer was so arranged as to read 140 scale 
divisions per volt. 
After testing the insulation the paraffined ball was charged 
positively and the rays played on it. After two minutes the 
electrometer reading was steady at 0°5 of the initial reading. 
The electrometer was then discharged by metallic connection, 
and again charged positively. Its reading remained steady after 
three minntes at 0°63 of the initial charge. In the third and 
fourth experiments the readings after three minutes were ‘SI 
and “90 of the initial charges respectively. 
The ball was next charged negatively. When the rays were 
played on it a steady reading was obtained after four minutes at 
‘18 of the initial charge. In the second, third, and fourth 
experiments the steady readings after four minutes were *45, 
‘70, and °78 of the initial charges respectively. 
The paraffin was then removed and the brass ball polished 
with emery paper; whether the charge was positive or negative, 
it fell in about five seconds to one definite position, 50 scale 
divisions on the positive side of the metallic zero, when the 
R6éntgen rays were played on the charged ball. 
These experimental results demonstrate that the Rontgen rays 
did not produce sensible conductance between the brass ball, when: 
it was coated with paraffin, and the surrounding metal sheath: 
and that ¢hey did produce zt when there was only air and no 
paraffin between them. From experiments by J. J. Thomson, 
Righi, Minchin, Benoist and Hurmuzescu, Borgmann and 
Gerchun, and Roéntgen,” we know that air is rendered temporarily 
conductive by Réntgen rays, and Rontgen’s comparison of the 
effect of the rays with that of a flame shows that our experimental 
results are explained by the augmentation of the electrostatic 
capacity (quasi-condenser) of the brass ball by the outside 
surface of its coat of paraffin being put into conductive com- 
munication with the surrounding lead sheath and the connected 
metals. 
In our second experiments we have endeavoured to eliminate 
the influence of the varying capacity of this quasi-condenser. 
For this purpose, we placed a strip of metal connected to the 
insulated terminal of the electrometer inside an aluminium 
cylinder ; the space between the metal and the cylinder was 
first filled with air, afterwards with paraffin. The aluminiym 
was connected to the case of the electrometer, and inductive 
disturbances were avoided by surrounding the copper wire con- 
necting the metal to the isulated terminal with a lead sheath 
in metallic connection with the electrometer sheath (see 
diagram). j 
In our first experiments with this apparatus we had air, 
instead of the main mass of paraffin, separating the insulated 
metal from the surrounding aluminium tube, as shown in the 
diagram, and we had only small discs of paraffin serving as 
insulating supports for the ends of the metal, and not played on 
by the Réntgen rays. When the metal thus supported was 
charged, .whether positively or negatively, the Rontgen rays 
1 By Lord Kelvin, Dr. M. Smoluchowski de Smolan, and Dr. J. 
Carruthers Beattie. Read before the Royal Society of Edinburgh, 
February 15, 1897. 
2J. J. Thomson, Proceedings R.S.L., February 13, 1896 ; Righi, Comptes 
Rendus, February 17, 1896; Benoist and Hurmuzescu, Comptes Rendus, 
February 3, March 17, April 27, 1896 ; Borgmann and Gerchun, E/ecrrician,. 
February 14, 1896; Rontgen, Wirzburger, Phys. Med. Gesellschaft, 
March 9, 1896; Minchin, The Léectrvictan, March 27, 1896. 
