234 
Transactions of the Royal Society of South Africa. 
on your tickets, but I venture to think a stranger visiting Cape, Town for 
the first time would prefer to have been provided with a map of the town 
which would indicate the position of this hall rather than to have to search 
the town for a room arranged according to such a plan. So with our 
photographs — the detailed study of each photograph can be of minor value 
without some indications enabling us to identify the field shown on the 
photograph not only in relation to the surrounding fields but in relation to 
the sky as a whole. It is certainly a desideratum that these indications 
•should be not less precise than those involved in the measures themselves. 
Of course if each photograph contained some familiar grouping of stars, 
such for instance as the constellation Orion, the identity of the region 
could not be easily mistaken, but the scale on which the plates are taken 
is such that two such familiar stars are rarely to be found within the 
limits of a single plate, while many plates are devoid of stars to which 
accurate positions could have beqn previously assigned. 
The necessity has thus arisen of determining with high accuracy 
the absolute positions as opposed to the relative positions indicated by 
the plate itself, of at least two stars contained on each plate. The 
distance between the images of these stars as compared with the 
corresponding apparent distance on the sky serves to furnish a deter- 
mination of the scale of the plate, while the direction of the line 
joining them serves to fix the orientation of the plate, i.e., enables us 
to turn the plate round in its own plane so that the true N. and S. 
line may be placed exactly in a truly horizontal, truly vertical, or, in 
fact, any other desired direction. With these quantities correctly 
assigned, either star will be sufficient to correlate the plate exactly 
wdth the sky. 
In practice, it is customary to refer to a larger number of stars 
than two on each plate. Not only are the determinations of these 
necessary " plate constants " strengthened thereby, but in the con- 
sistency with which the photographic pictures reproduce the actual 
distribution of the stars, we have a valuable check on our methods 
and a test of the faithfulness with which also other features on the 
sky are represented on the photographs. 
For the Cape plates, the determination of the ''plate constants," 
i.e., of those elements which serve as it were for the co-ordination of 
the isolated maps contained on each plate with a general map of the 
whole has been made to depend on a number of stars on each plate 
varying as a rule between 8 and 12. In all, 8,560 stars, as far as 
possible evenly distributed over the whole of the zone, were selected 
as comparison stars. These stars constitute a framework to which 
the detailed results derived from the photographs may be attached, 
but the structure of which requires to be independently built up. 
