THE YOUNG 
persons writing from one part of Paris to the 
other might have their letters, etc., faitlifully 
carried and diligently delivered to any address, 
provided they attached to their letters a ticket 
indicating that its carriage was paid, as money 
would not be directly accepted. This ticket 
might be attached to or placed around the 
letter, or aflBxed in any convenient manner so 
that it be plainly visible. The price of this pre- 
paid ticket was a sou. The system had been in 
force since August, 1633. 
A Larg-e Object-Glass. 
The object gbiss for the great refracting tele- 
scope intended for the Russian observatory of 
Pulkowa, has been finished by the Messrs. Clark, 
of Cambridge, Mass., and is the largest glass 
thus far constructed. The order for the glass 
was given in 1881 by Prof. Struve, the director 
of the observatory, who has lately visited this 
country for the purpose of subjecting the glass 
to certain tests of its quality. These, it is said, 
proved to bo in every way satisfactory, and the 
object glass is by this time on its way to its ulti- 
mate destination, if, indeed, it has not already 
arrived there. This great object glass is thirty 
inches in diameter. Pulkowa, however, will not 
be able to claim very long the distinction of hav- 
ing the largest refracting telescope in the world, 
for the great Lick telescope for California will 
have an object glass of thirty-six inches diam- 
eter, or six inches greater. Six inches appears 
a small addition, but it represents an enormous 
increase in power, for the liglit-gathering area 
of tlie Lick object-gJass will be to tliat of Pul- 
kowa as 1,296 to 900, or forty per cent, greater. 
The rough disk of the flint portion of the Lick 
glass has already been receiv(Hl by the Clarks, 
and tlKiy are at work upon it. It will be several 
years before it is finished. 
Advances in Scientific Discovery. 
The opinion is commonly expressed by those 
who are not well informed, that the world is so 
rapidly advancing in scientific discovery that a 
few years more of such extraordinary advance- 
ment as we have witnessed during the past 
quarter of a century, will practically exhaust 
the sources of discovery, and the scientist will 
sigli in vain for " for new worlds to conquer." 
No opinion could, in reality, be more delusive 
than this. Each fresh discovery which discloses 
a secret that nature has so long and carefully 
guarded, illuminates with its feeble rays a back- 
ground of other mysteries still unsolved, and 
so far from narrowing the field of discovery, 
widens the circle of the unknown and places its 
horizon still further off. He who thinks he has 
mastered a science is a superfici;il thinker. The 
SCIENTIST. 351 
profound thinker is he, who, having possessed 
himself of all the knowledge of his predecessors, 
perceives how little the sum of tliat knowledge 
is, in comparison with what still remains to be 
learned. 
—There is a curious clock iu Japan. This clock, 
in a frame three feet high and five long, repre- 
sented a noon landscape of great lovliness. In 
the foreground were plum and cherry trees and 
rich plants in full bloom; in the rear, a hill 
gradual in ascent, from which flowed a cascade 
admirably imitated in crystal. From this a 
thread-like stream glided along, encircling in its 
winding rocks and tiny islands, but presently 
losing itself in a far-off stretch of woodland. In 
the sky turned a golden sun, indicating as it 
passed the striking hours, which were all marked 
on the scale below, where a slowly creeping tor- 
toise served as a hand. A bird of exquisite plum- 
age, resting by its wings, proclaimed the expira- 
tion of each hour. When the song ceased a 
mouse sprang from a grotto near by, and, run- 
ning over the hill, hastily disappeared. 
— " C. S.," says an Exchange, " asks the origin of 
the belief or superstition that it was fortunate for 
any one to find a horse-shoe and that nailing it 
over one's door or hanging it in the house would 
bring good luck. The Eiiglisli antiquary, John 
Aubrey, who wrote about the middle of the seven- 
teenth century, says that in his time most of the 
houses in the West End of London were pro- 
tected against witches and evil spirits by having 
horse-shoes fastened to them in various ways. It 
was the belief that then no witch or evil genius 
could cross the threshold which was protected by 
the shoe. The fact is that the superstition has 
been traced so far back, and then we lose it in the 
obscurity of the ages. The custom of nailing 
horse-shoes for luck to all kinds of sailing craft 
is still in vogue, and is religiously maintained to 
be a wise and lucky measure. The superstition 
goes further, by making it fortunate for any one 
to find a horse-shoe, and the good luck is in- 
creased with the number of nails that are at- 
tached to the shoe when it is picked up." 
— Cros. Yergeraud has utilizf^d the properties 
of bichromates to produce positives directly on 
paper. He first saturates a suitable kind of paper 
v\^ith a solution of 2 parts of bichi-omate of am- 
monia and 15 of glucose in 100 parts of water, 
dries it, and exposes it under any positive (either 
a glass transparency, a drawing, tracing or other 
flat object). As soon as the exposed parts turn 
gray, it is immersed in a bath consisting of one 
part of nitrate of silver and 10 of acetic acid in 100 
parts of water. The picture makes its appear- 
ance at once, and is of a blood-red color (bi- 
chromate of silver). Wherever the light acts 
upon it. the glucose reduces the bichromate, but 
in those places which have been protected by the 
