232 Transactions of the Royal Society of Sonth Africa. 
in Paris in 1887 to discuss the possibility and desirability of carrying out 
this extended project. 
While the plan in outline met with universal acceptance, much useful 
discussion took place at this and at subsequent conferences as to the details 
of its execution, such as the type of instrument to be employed, the scale 
of the photographs, the duration of exposure which would serve to bring 
out what should be considered essential detail, &c. 
The outcome was that an agreement was arrived at by which the 
execution of the work was partitioned among eighteen co-operating Obser- 
vatories, who between them undertook to secure photographs of the whole 
sky with instruments of similar dimensions and design, and as far as- 
possible similar conditions of exposure. Two series of photographs were 
to be taken, one of long exposure designed for direct photographic repro- 
duction to form an atlas which should show all stars down to the Mtb 
visual magnitude, and a second series, of shorter length of exposure 
designed for exact measurement, which would form the basis of a catalogue- 
of precision of all stars to the 11th magnitude. Minor details were largely 
left to the discretion of the directors of the co-operating Observatories. 
It would take too long to describe in detail the progress of this work 
from its initiation, and I propose, therefore, to confine my attention to the 
share in it which has been taken by the Observatory over which I have 
the honour to preside — a share which, partly from the origin of the project- 
and partly from the scarcity of suitably situated observatories of a suffi- 
ciently permanent character in the Southern Hemisphere who were 
able to offer co-operation, has exceeded that assigned to any other single- 
establishment. 
The type of instrument agreed upon was the photographic refractor 
similar in dimensions to that which had been used by the Brothers Henry 
in Paris, giving pictures on such a scale that one millimetre on the plate 
corresponds with a minute of arc on the sky. The plates were to be 
160 mm. square, so as, apart from the marginal edges, to yield an effective- 
field of view of two degrees square. The area covered by each plate will 
perhaps be better presented to you by the statement that the length of 
the edge of the plate corresponds roughly with four times the apparent 
diameter of the sun or moon, i.e., four suns or four moons photographed 
side by side on the same plate would just reach from one edge of the plate 
to the other. On this scale it would require rather over 10,000 plates tO' 
cover the whole sky. The programme, however, provided not merely for 
the photographing of each region once and once only, but that the various- 
regions covered by the separate photographs should overlap in such a way 
that every star would be contained in two of the regions at least. Thus- 
any doubtful features shown on one plate could be verified by reference to< 
the second plate which includes the same area. 
