Photography mi Aid to Astronomy . 
233 
This requirement raised the total number of separate regions to 
be photographed to 22,054, of which 1,512 were assigned to the Cape 
Observatory. 
The construction of the telescope and the provision of suitable housing 
accommodation were sanctioned in 1888, and the telescope arrived at 
the Cape in 1890. About two years was spent in its erection, adjustment, 
and testing, the delay being largely due to the necessity for returning the 
object-glass for necessary alterations and the remodelling of the breech- 
piece which could not be effected locally. From July, 1892, onwards, the 
work of taking the photographs was steadily proceeded with until its com- 
pletion in 1910. 
At the present time we have stored at the Cape three almost complete 
series of photographs of the 1,512 regions besides many duplicates and 
triplicates, which, though not attaining the desired standard of perfection, 
are still available for reference with regard to many issues that arise. 
Of these plates one series of long exposure is destined for direct photo- 
graphic reproduction ; the other two series of shorter duration of exposure 
or measurement for the purposes of the catalogue. On account of the 
comparatively insignificant amount of work involved in taking these 
catalogue plates as compared with the more laborious work of measure- 
ment, after its original completion in 1896, it was decided to repeat the whole 
series of catalogue plates so as to bring the dates of the photographs nearer 
to the epoch 1900 and thereby more fully attain the ideal before the Inter- 
national Conference of leaving for future generations as exact a record as 
possible of the appearance of the heavens at the beginning of the twentieth 
century. 
After some early experiments with a view to ascertaining the most 
economical means by which these plates might be measured with the 
desired accuracy, the definitive programme of measurement was commenced 
in 1897 and completed last year. The number of stars measured on a 
single plate varies between 45 and 3,431, the total number of star images 
which have been measured exceeding a million. To illustrate the precision 
attained, the discordances in the position of a star, relatively to the stars 
immediately surrounding it, as derived independently from the measures 
made on two overlapping plates, but rarely amount to as much as a second 
of arc, i.e., apparent angular distance between the two edges of a half- 
penny at a distance of about three miles from the observer. 
Now, however exactly the measurable features of each plate may be 
determined, it will be clear that the information thus derived can be of 
little value without some external data to enable the identification of the 
region on the photograph with the corresponding region of the sky from 
which it is derived. A plan of this room might have been useful to enable 
you to find your seats to-night in positions corresponding with numbers 
