LT AnS IN WVe PAG SCZ 146 Mar. 20, 
But not only are we able, by the aid of photography, to study the mem-, 
bers of the solar system, but it is possible thus to investigate the rela- 
tions of the immensely far-off stars, and even to determine their distances. 
In the heavens we discover with a telescope that many stars are 
changing their positions and distances with respect to their near visible 
companions ; and investigations have been made proving that some of 
these revolve about the others as centres, forming systems of their own, 
like our solar system. 
The measurements required for this work are numerous and delicate, 
and can be made only on nights when the seeing is very fine, by a skill- 
ful observer. Mr. RUTHERFORD has adopted a method of photo- 
graphing a cluster of stars, or a double star, in such a way, that the 
plates can be laid away and measured by a person simply trained in 
the use of a micrometer. These plates may be taken at large inter- 
vals. On comparing the measurements, a decision can be reached as to 
whether any stars have moved with respect to the others. 
Let me describe Mr. RUTHERFORD’S method : 
First, a wet plate is exposed for four minutes. This gives stars 
down to the tenth magnitude. But there may be points on the plate 
which are not stars; hence a second impression is taken on the same 
plate, after it has been slightly moved. All points now doubled are 
true stars. Now for measures of arc. After this second photograph 
is taken, the clock is stopped. The now moving stars down to the 
fourth magnitude are bright enough to leave a continuous line; the 
length of this, in a very accurately known interval, say two minutes, 
enables the arc to be calculated. Next comes the mapping. The 
negative is fixed on a horizontal divided circle on glass, illuminated 
from below. Above it isa system of two rails, along which travels a 
carrier with two microscopes, magnifying fifty diameters. By the one 
in the centre, with two cross-wires in the field of view, the photograph is 
observed. By the other, armed with a wire micrometer, a divided scale, 
on glass, which is fixed along the rail, is read. Suppose we wish to meas- 
ure the distance between two stars on the plate. The plate is rotated so 
that the line which joins them coincides with that which is described 
by the optical axis of the central microscope (marked by the cross- 
wires), when the carrier runs along the rails. This microscope is then 
brought successively over the two stars, and the other microscope over 
the scale reads the nearest division. Hence then, the fixed scale, and 
not a micrometer screw, is depended upon for the complete distance. 
In this way the distance between two stars on the plate can be meas- 
ured to the ysheoth part of an inch. 
By the measurement of such plates, we may hope to add a great 
deal to our knowledge of the stellar systems. 
