ON THE TOTAL SOLAR ECLIPSE OF MAY 17, 1882. 
255 
Both collimator and camera had a focal length of about 9 inches; the prism had a 
refracting angle of 62°, and an aperture of 2 by 1 f inches. The sensitive plate in 
this camera could be tilted with the axis of the lens in the manner already explained, 
and the spectrum is in fair focus throughout the range of sensitiveness of the plate. 
In fixing the time of exposure for each plate and camera we had to consider the 
probable intensity of the light, but we had also to take into account the loss of 
time involved in changing the plates. At first it seemed, indeed, as if 12 seconds 
would be required to change each slide, but after a good deal of practice the time 
thus lost was finally reduced to 7 seconds, though even this was a considerable 
proportion out of the total of 70 seconds during which totality was to last. It 
was therefore decided that only one plate should be exposed in each of the spec¬ 
troscopic cameras ; but that three photographs of the corona should be attempted 
with the large lens. A trial was also to be made to obtain an impression of the 
first flash of light at the end of totality with the prismatic camera. A detailed account 
of the times of exposure of the different plates actually obtained will be given under 
the head of “ Results.” 
It was arranged that Mr. Woods, who was the assistant for the photographic work, 
should, during totality, cover and uncover the different lenses at the proper times, 
whilst I undertook to change the slides. Mr. J. Y. Buchanan was kind enough to 
call out the time at intervals of 10 seconds during the whole of totality. An arrange¬ 
ment of this kind is absolutely necessary where plates have to be changed, and wdiere 
each second is of importance. 
The telescope was mounted and adjusted in the usual way ; the adjustment for 
latitude was not perfect, and a firmer foundation would have been desirable, but as a 
very slight motion was of little consequence in the spectroscopic cameras, while 
the longest exposure for the corona was only to last 22 seconds, even a rough 
adjustment would have been sufficient; and the results were in no way affected 
by this want of adjustment or by any irregularity in the motion of the clock. The 
final adjustment of the clock was made only a few minutes before the beginning of 
totality, and its behaviour during the critical time was better than could have been 
expected from its previous performance. 
In order to fix the position of the corona on the plate from its photographs, a 
platinum wire was stretched across the camera close to the sensitive plate. Before and 
after totality the clock was stopped, and photographs of the solar cusps were taken at 
an interval of 2 minutes on the same plate. In this way the position of the platinum 
wire could be ascertained relative to the sun’s path directly before and after totality. 
The accurate orientation of the solar corona in the sky is a very important matter, 
and one which has been too much neglected in some of the recent eclipses. 
All the plates were gelatine plates, specially prepared by Captain Abney for the 
occasion. Those used in the prismatic camera, as before stated, were sensitive in the 
red as well as in the blue. From calculations made, it appears that all the plates used 
