22 
at one period certain lines were shifted towards the violet end 
and at another time towards the red. He concluded, therefore, 
that the star was moving towards us in the first case and away 
from us in the second. By the application of this method, and 
by the perfection of the instruments used in measurements re- 
quiring such care and exactitude in which even a hair’sbreadth 
is a matter of moment, astronomers have been able not only to 
compute the orbits of suns which are visible to us, but even to 
infer the existence of the obscure companions of their course and 
to describe the invisible. 
But the part which the camera has played in stellar 
photography is perhaps even more remarkable than the telescope 
has done. In 1882, Dr. Gill, of the Cape Observatory, essayed to 
obtain a photograph of the comet visible in that year by means 
of an ordinary portrait lens of 2 in. aperture. To his surprise he 
saw on developing the plate the impression of stars not visible 
even with a comparatively large telescope. The news of the 
discovery soon brought many enthusiastic workers into so 
promising a field of investigation. The excellent work of 
Pickering in America, of Pritchard in England, and the Brothers 
Henry in Paris, soon obtained notoriety. At Dr. Gill’s suggestion 
a conference of astronomers was convened at Paris in 1887, and 
then it was decided, by the co-operation of astronomers in all 
countries, to commence a photographic chart of the heavens. 
Argelander’s great map which gives the magnitude and 
position of all stars down to the ninth magnitude, which are to 
be found in the track of sky extending from the North Pole to 
about two degrees south of the Equator, contains 320,000 stars. 
Dr. Gill has very recently sent home, as the first instalment to 
his work on the photographic chart, a plate so small that it 
would be hidden by a shilling held at arms length, yet containing 
no less than 50,000 stars. 
