PRESIDENT’S ADDRESS. 161 
The latter telescope is necessary in order to ascertain that the 
region under exposure is accurately followed. 
The photographic plates are 158 mm. square, and cover an area 
of more than 6 degrees, of which 4 degrees only are used. 
In order to define the centre and the orientation of the plate in 
respect to the region photographed, a latent image of a reticule 
is impressed on the plate before exposure. 
This consists of a set of parallel lines at intervals of 5 milli- 
meters, crossed at right angles by another similar set of lines. On 
developing the plate “after exposure to the sky, an image of the 
reticule is thus permanently secured. 
The intersection of the central lines is taken as the centre of 
the plate, or of the region photographed ; one of the central lines 
represents the path of an equatorial star, and the other central 
line at right angles to it consequently corresponds to a celestial 
meridian. 11,027 plates are required to cover the whole sky 
once, and the sky is accordingly subdivided for this purpose 
into as many elementary areas, which are photographed one by 
one. 
The plan decided by a Special Commission of the Permanent 
Committee for carrying out these operations was as follows :—- 
Imagine parallels of declination to be traced on the celestial 
sphere, 1 degree apart, from zero at the Equator, to 90 degrees at 
the Poles. Then on and along the parallels of even degrees, mark 
equidistant points at intervals of 2 degrees of a great circle. 
These points correspond to the centres of the elementary areas. 
which, in the photographs, are represented by the intersection of 
the central lines of the reseau. 
The 11,027 pictures thus obtained would comprise the whole 
extent of the heavens. As we have seen above, it was prescribed 
that the entire surface should be photographed a second time, 
both for the short exposure and long exposure plates. 
In the second series, instead of photographing the same elemen- 
tary areas as in the first, the centres were marked on and along 
the parallels of odd degrees of declination at intervals of 2 degrees 
of a great circle, so arranged as to be coincident with the point 
of junction of four adjoining plates of the first series. 
Thus the stars which form their image near the corners of one 
plate will also appear near the centre of another plate, which is 
one of the many advantages of the method. 
An area of 4 square degrees covers a square of 120 mm. on 
the plates ; but the outer lines of the reseau enclose a square of 
130 mm.; the photographs do, therefore, overlap to the extent of 
5 mm. on each side. 
The plan of distribution and arrangement of the centres of the 
elementary regions is the same in both the catalogue, or short. 
exposure plates, and the chart, or long exposure plate Adii A i 
o 
