THE YOUNG SCIENTIST. 
45 
artistic purposes. According to the time of the 
immersion, the strength of the current and the 
proportion of copper to the tartaric acid, the ir- 
idescences may be produced of different shades 
and tints, which may be varied or intermingled 
by sliielding certain parts of the object by an 
impermeable coating of parafflne or varnish, 
while the iridescent effect is being produced on 
the parts left exposed. All colors, from that of 
brass to bronze, scarlet, blue, and green, may 
thus be produced at will.— Me^ai Worker, 
An Artificial Sun. 
The most powerful artificial light in the world 
has just been constructed by Messrs. Chance 
Bros. & Co., at Smethwicli, near Birmingham, 
Eng., for the South Head Lighthouse, near Syd- 
ney, New South Wales. It is a first order diop- 
tric, revolving light, with the electric arc. The 
lamp has a special arrangement of prisms for 
securing vertical divergence of the beam. It is 
over six feet in diameter, and the height is 
about nine feet, and it is said to be the first time 
such dimensions have been applied to illumina- 
tion by the electric arc. The lamp has a power 
of about 12,000 candles in the focus of light, and 
the merging beam has a luminous intensity ex- 
ceeding 12,000,000 candles. The light will give 
flashes around half the horizon at intervals of a 
minute, and wull make a complete revolution 
every 16 minutes. On an average, the light will 
be visible at a distance of 40 to 50 miles- At an 
exhibition of its power recently given at Smeth- 
wick, the light was so intense that it could 
hardly be endured by the naked eye. 
Iridium for Electric Lamps. 
A promising improvement has been made in 
electric lighting by substituting electrodes of 
iridium for the carbon pencils hitherto almost 
exclusively used as poles in exhibiting the vol- 
taic arc. Iridium is fusible only in the arc from 
a very powerful battery, and by keeping the in- 
tensity of the current below a given point, the 
metal can be maintained at a temperature of 
about 4,000° F., which is far below its melting 
point, though high enough to add the light of 
an intense incandescence to the brilliancy of the 
arc itself. At this temperature the iridium 
points remain completely unchanged, and the 
variations and flickerings due to the rapid wear- 
ing away of carbon pencils are entirely absent 
in the new light; while the consistency with 
which the source of illumination keeps its place 
peculiarly adapts it for use in the focus of the 
Fresnel lenses, or the parabolic reflectors which, 
in lighthouses, or in similar situations, take 
their place. The process by which the iridium, 
naturally a gritty and intractable powder, is 
formed into pencils for use in lamps is an ingC' 
nious adaptation of a patent issued long ago for 
making the same metal into points for gold 
pens, and depends upon the property which ir- 
idium possesses of forming a compound with 
phosphorus, which can then be melted at a 
temperature of about 3,000^ F.— about the fusing 
point of iron— and molded into masses whicii 
can be subsequently dephosphorized, and regain 
the original infusibility of the pure metal. 
The Largest Refracting Telescope in 
tlie World. 
For the present our friend, Alvan Clark, is 
outdone by his Hibernian rival, Howard Grubb. 
The 26-inch refractor at Washington, the largest 
previously made, must yield the palm to the 
27-inch one lately finished at Dublin for the 
Eoyal Observatory at Vienna. 
It is a curious tact that the Emerald Isle 
should have produced the largest refractor as 
well as the largest reflector, which our readers 
are aware is Lord Eosse's. The former was or- 
dered by the Austrian Government in 1875, and 
the mechanical portions of the instrument were 
nearly finished in 1878. The making of the ob- 
ject-glass was delayed by the difficulty of get 
ting perfect material. Messrs. Fell, of Paris, 
furnished several sets of glass discs for the pur- 
pose before any were found free from flaws and 
defects. Much time and labor were lost in work- 
ing some of these discs into such shape that 
their imperfections could be detected. It was 
not until March of the present year that the in- 
strument was complete in all respects, and its 
performance approved by the commissioners 
appointed by the Austrian Government to test 
it. 
The tube of the telescope is made of steel 
plates riveted together, and measures 33| feet in 
length, with a diameter of SQ^ inches in the mid- 
dle, tapering to 27 inches at one end and 12 at 
the other. The weight of the tube with its at- 
tachments is between six and seven tons, but 
the whole weight can be easily moved by the 
observer with one hand. The great dome in 
which the telescope is to be placed is also of 
steel, consisting of a double shell of thin plates 
with light ribs and girders between, the whole 
forming a cellular structure somewhat like that 
of the top and bottom of the Britannia Tubular 
Bridge. Great strength and stiffness are thus 
secured, while the weight is reduced to the 
minimum. The dome, 45 feet in diameter, 
weighs only about fifteen tons, and when set up 
on temporary supports at Dublin could bo 
moved round with a tractive force of sevent>- 
