186 E. H. BOOTH. 
The solar image had a diameter of 1°7 inches, and the 
equivalent stop was f 64. ; | 
(2) An aeroplane camera of focal length twenty inches 
and stop f 6°3, with an adjustable K 3 filter. The camera 
was equatorially mounted. 
This instrument was lent for the purpose by the Military 
Board, and was adapted by the Hxpedition to give time 
exposures, and to feed as a magazine camera pointing up- 
wards instead of downwards. The plates were Ilford 
Special Rapid panchromatic, size 5 x 32 inches. 
(3) A box camera specially constructed for this work, of 
focal length forty-five inches and stop f 13, equatorially 
mounted with and alongside the aeroplane camera. The 
lens was a rapid retilinear, and should apparently have 
been stopped down, as the eclipse image was not as sharp 
as trial plates had indicated. The plates employed were. 
Imperial Special Sensitive, backed. 
The first difficulty experienced was to obtain some idea 
as to the relative time of exposures for inner and outer 
corona, and the absolute time of exposure for either of them. 
A careful search of earlier records showed that the time of 
exposure varied considerably with different writers, and 
that the same exposure at different eclipses appeared to 
give widely different results. It was decided, therefore, 
to work on the principle that provided the matter required 
was on the plate, i.e., provided a sufficiently long exposure 
was given, it could be handled as required in developing 
or in printing. Consequently full exposures were to be 
desired, the limitations being the necessity of exposing 
several plates for different effects, and the possibility of 
inaccuracy of movement of the coelostats or equatorial 
mounting. The mean exposure decided on was the equi- 
valent of 10 seconds at f 8 for outer corona work, and 0°5 
second at f 8 for inner corona work. When a number of 

