1872: ] | Light. 119 
is clear that their velocities relative to the earth are the same, since both are 
moving perpendicularly to the line of vision; the lines from the two stars will 
therefore coincide. But when their apparent distance from each other is 
greatest, the difference of their velocities relative to the observer is equal to 
the velocity of either star, in its velocity in its orbit about the other. This 
difference of relative velocity will produce a displacement of the lines, which 
displacement may be observed and even measured. This gives the value of 
that velocity; but we know also the periodic time. We have, then, at once 
the circumference, and thence the diameter of the orbit. We know the 
greatest angular distance between the stars; we have, then, the distance of 
the stars from the earth. 
M. Papafy has devised a series of sky-rockets adapted for telegraphic 
service at night for armies when in the field, so arranged that each rocket is, 
by a variation of coloured light, capable of transmitting six words, visible at 
a distance of twenty English miles. These signals can be readily kept un- 
intelligible to the enemy, while everything relating to military and strategical 
matters can be easily expressed. The Prussian War Department has bought 
the secret of this invention from the author, a Hungarian in service as captain 
in the United States Army. 
W. Miiller and Dr. F. Knapp have published a very exhaustive memoir on 
that kind of glass which owes its beautifully red-purplish colour to gold. The 
first portion of this monograph contains a review of the literature of the 
subject alluded to, and of the various theories and opinions held on the con- 
dition of the gold while acting as a pigmentary or staining matter. It appears 
that the quantity of gold required to impart eolour to the glass mass is very 
small indeed, since 1 part of gold in 100,000 of the metal (as the molten glass 
mass is technically called) distinétly yields a rose-red colour. The authors 
did not succeed in discovering by experiments what the condition is of the 
gold in the glass; the chief reason of this failure is that the quantity of gold 
is almost infinitesimally small. 
At last there appears more than a probability that the oxyhydrogen light may 
be employed with financial success. The experiments recently instituted at 
the Crystal Palace at Sydenham with M. Tessie du Motay’s apparatus are in 
every way satisfactory—that is, scientifically, for the statements as to economy 
must at present be based only upon calculation. The apparatus employed is 
extremely simple and may be described as follows:—A small one-cylinder 
engine drives a set of three small air-pumps at a rapid rate; these pump air 
into two retorts built up in a furnace, the external walls of which are about 
1o feet long by 6 feet by 6 feet in section. These retorts are charged with 
manganite of soda, mixed with some oxide of copper. The manganite has a 
great affinity for oxygen and takes a large percentage out of the air, leaving 
the nitrogen to escape by small iron pipes. The manganite is thus saturated 
with the oxygen, which is carried over into a reservoir by a blast of superheated 
steam, the manganite remaining free to again absorboxygen. In the reservoir 
the steam condenses into water, leaving the reservoir filled with oxygen. By 
means of the alternation afforded by the two retorts the process becomes con- 
tinuous, and as fast as the air is pumped in so the oxygen is regularly given off. 
In this way 1 cubic foot of oxygen is the product of 20 cubic feet of air and 
15 of steam. The burner is in shape like a double ordinary burner with one 
tap for the oxygen and one for the hydrogen. The top of the burner is dished 
out into a hemispherical cavity, in the centre of which is the oxygen hole, and 
surrounding it some eight or ten smaller holes for the hydrogen. There is 
thus obtained a solid cone of light. The equivalent of 5 cubic feet of ordinary 
gas burnt in the usual manner is, in the new light, 1 cubic foot of the same 
coal gas with about ths of a cubic foot of oxygen. At too yards from the 
candelabra of twenty lights in the centre transept of the Palace small hand- 
writing can be easily read. 
Mr. John Browning has favoured us with a record of the steps he has 
taken towards the introduction of compound prisms. In 1864 he made for 
Mr. Gassiot a very powerful battery of bisulphide of carbon prisms. Instead 
