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CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
V. The Photographic Camera. By Arthur Schuster. 
I. Adjustments. 
The lens used for obtaining photographs of the corona had a clear aperture of 
4 inches (lO'l cm.), and a focal length of about 5 ft. 3 in. (160 cm.). It therefore gave 
images of the Sun of about '3 in. in diameter. 
The accurate adjustment of the camera is a matter of some difficulty, as distant 
objects are not sufficiently sharp to admit of delicate focusing. It was found better 
to focus on a sharp object at a moderate distance, and to find by calculation the 
correction for parallel rays. The correction to infinity for a distance u of the object 
focussed is given by 
hf = Plu. 
If, for instance, u were 1 kilometer, and f, as mentioned above, 160 cm., the correction 
would be 2'6 mm., and, as it proved impossible to adjust the focus more nearly than 
to about half a millimeter, an error of 20 per cent, in the estimate of the distance is 
allowable. 
It is a matter of importance to be able to determine accurately the position of the 
corona in reference to the coordinates fixed in space. In Egypt and at Caroline Island 
this was done by means of a platinum wire stretched across the camera directly 
in front of the plate ; the shadow of this wire appears on all photographs, and its 
position was determined by a succession of instantaneous photographs of the Sun’s 
crescent taken directly before and after totality with a stationary telescope and at 
short intervals of time. Tins plan has some disadvantages. The shadow of the vdre 
hides certain parts of the corona, and, for instance, in the photograph taken at 
Caroline Island the shadow is very awkwardly placed. Then, again, it is difficult to 
stretch the wire sufficiently tight. The camera has to be turned directly on the Sun 
while the crescent is being photographed, and tlm heat thus concentrated on the wire 
lengthens it and may change its position. 
A different plan was therefore adopted. Two needles were placed one on each 
side of the camera, the line joining the points passing approximately through the 
centre of the plate ; one of the needles wms intentionally inserted somewdiat obliquely, 
so that its shadow might always be recognised on the photographs. The image of 
the needles would thus completely determine the position of the plate in the camera. 
Tlie direction of the line joining the needle-points can be very accurately determined 
with reference to the declination circle. We need only bring the image of some 
object, either terrestrial or celestial, just in coincidence with one needle-point, then 
with the other, and note the change of readinof of the hour and declination circles. 
For safety, and as a check, photographs of the Sun’s crescent might be taken after 
totality. The needles, in the present instance, did not reach sufficiently far into the 
centre of the camera. In previous eclipses the general illnmination of the sky in the 
