TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893. 
563 
middle of the telescope tube, the shutter was then opened to its full extent, and an 
exposure made; the plate in the second compartment was next brought to the 
middle of the frame, by pushing the slide along, and also exposed ; again, by moving 
the slide along, the third plate was brought into position and exposed, after which 
the shutter was closed and the slide withdrawn. During the exposure of any one of 
the three plates in a slide, the other two were protected from light by the rectangular 
frame. 
The upper edge of each dark slide was notched in three places, corresponding to the 
positions of the three plates which it contained, and, as each plate came to the 
proper position for exposure, as the slide was pushed along, a spring catch auto¬ 
matically dropped into its place. 
Upon the back of each dark slide six numbers were painted in clear white figures. 
A small series of numbers corresponded to the numbering of the 30 plates to be 
exposed during the eclipse, and a larger series indicated the exposures to be given to 
each plate, so that it was unnecessary to refer to any list. 
These time-saving devices are of the highest importance in eclipse work, and too 
much attention cannot be given to them. The arrangements in West Africa worked 
admirably, and it was possible to change from one plate to another in about a second 
when a slide was once inserted, and to change the whole slide in 5 seconds. Longer 
intervals, however, were allowed to elapse between the exposures, in order that the 
instrument might steady itself, and to correct the backlash of the driving screw. 
Method of Focussing. 
The instrument was focussed by photographing the spectra of some of the brighter 
stars. This is the only satisfactory method of focussing the prismatic camera, as rays 
from a star fall on the prism under exactly the same conditions as those from the 
eclipsed sun. If a slit and collimator be employed, identical conditions can only be 
obtained when the collimator is perfectly achromatic and absolutely adjusted for 
parallel rays. 
From C to the extreme ultra-violet the focus is sufficiently constant with the 
Henry lens, to enable the use of a swing back to be dispensed with, and this is an 
immense advantage in work with the prismatic camera, for the reason that opposite 
sides of the rings of light corresponding to the various radiations will be equally well 
in focus. 
The Plates Employed. 
There was a little uncertainty with regard to the kind of plates which would be 
most suitable in the climate of West Africa, and four different commercial brands 
were therefore taken out, namely :— 
4 c 2 
