PhotograpJiy an Aid to Astronomy. 
231 
no light task, and might well give rise to the question as to whether the 
necessary measurements could not be more advantageously made on the 
«ky rather than on the photographic plates by which it was represented. 
The method which was subsequently adopted for the formation of this 
■catalogue forms sufficient answer to this question. The resources of the 
"Observatory were unequal to so large an undertaking, at least without the 
<jessation of a large proportion of its more normal work, and it appeared 
probable that the photographs would have to remain in the record room of 
i;he Observatory, valuable for consultation regarding specific points which 
might arise, but that their complete discussion would have to be relegated 
to a perhaps remote future. The difficulty was at this juncture met by 
a generous offer received from Professor Kapteyn, of Groningen. Pro- 
fessor Kapteyn was himself an enthusiastic astronomer, eagerly desirous 
of devoting himself to work of this character, with full appreciation of its 
value, but unfortunately unprovided with an Observatory equipment. He 
foresaw that, with the aid of a comparatively small laboratory equipment, 
much of the work of a character which had hitherto been performed by 
reference to the sky could be effectively and rapidly carried out through 
the medium of photographic representations thereof. The series of photo- 
;graphs secured at the Cape formed a promising field for research, and he 
voluntarily undertook the laborious task of the measurement of these 
plates and the preparation of a catalogue, which has since been issued 
^mong the publications of the Cape Observatory and which to-day is in 
<jonstant use by astronomers concerned with the Southern Hemisphere. 
Besides many incidental results of high interest, the catalogue forms the 
foasis for statistical investigations of stellar distribution, &c. 
The success of the project even in the early stages of its execution 
pointed to the possibilities of a still more extended use of the photo- 
graphic methods, and gave a direct stimulus to the study of the details by 
which its efficiency could be increased. The surveys of Argelander and 
Schonfeld completed by the " Cape Photographic Durchmusterung " (as 
the above catalogue is entitled), and independently by a visual " Durch- 
musterung " emanating from the Observatory at Cordoba under the 
■direction of Dr. Thome could but be regarded as preliminary pioneer 
surveys ; but stupendous as the task seemed the possibility was recognised 
toy Sir David Gill of utilising the photographic method for the production 
of a complete map of the heavens which should be on a sufficiently large 
scale and sufficiently precise in its detail to meet all the requirements 
of modern astronomy of precision. 
He succeeded in securing the interest of astronomers generally and 
lin particular of Admiral Mouchez, the then director of the Paris Obser- 
vatory, at whose instance, with the support of the Paris Academy of 
•Sciences, an international conference of astronomers was invited to meet 
