ON THE TOTAL SOLAR ECLIPSE OF AUGUST 29, 1886. 
319 
line in the spectrum. Thus, in a photograph taken during totality, there will he 
several distinct images of each prominence, and, in measuring the distances between 
them, we are, in fact, measuring the distances between the corresponding lines in the 
spectrum of the prominence. A number of small prominences close together gives 
the appeEirance of a segment of a circle, while the continuous spectrum of the corona 
shows as a confused band across the plate. This wuh explain why it is difficult to 
obtain a clear interpretation of the various appearances shown in these photographs. 
The accurate adjustment of the prismatic camera in its present form is difficult. 
In order that the photographic plate may be in focus for a large range of the 
spectrum, the camera is provided with a back which can be inclined to the optic 
axis of the instrument. In order to find the proper angle and position of the plate. 
Sun light which had passed through a collimator accurately adjusted for parallel rays 
was thrown into the instrument. The spectrum thus formed was focussed on the 
plate, and the instrument, therefore, adjusted for parallel rays entering in the 
direction of the optic axis of the collimator. The position of the blue and red end of 
the spectrum was then marked on the ground glass of the camera, and the collimator 
removed. If the instrument were then fixed on the equatorial, it would have been in 
focus, provided the camera w'ere pointed relative to the Sun as it wms before relative 
to the collimator. This wms the awkward part of the adjustment; the camera was 
turned until the spectrum formed by the Sun occupied the position previously marked. 
The spectrum, of course, was blurred, owing to the finite size of the Sun, but the 
adjustment could be made approximately, and the result shows that fair definition 
was obtained. The slant of the plate essential for securing fair focus for rays of 
different refrangibility mmst be detrhnental to the simultaneous focus of different 
parts of the Sun’s circumference. The instrument can, therefore, never be a perfect 
one, but it affords valuable inforjnation in every eclipse in which a number of sharply 
defined prominences appear. 
2. Observations and Residts. 
Only one photograph was taken during totality. The exposure was estimated at 
nearly two minutes, but for about fifty seconds the corona was nearly obscured by 
clouds. It has been explained how other photographs with another instrument, but 
on the same mounting, had to be taken at the same time as this one, and how it, 
therefore, came about that the plate shows signs of a shake. 
The images of five prominences can be distinguished, but of these only twm were 
measured, which will be called prominences Nos. I. and II. No. I. is evidently the 
large prominence on the north-west of the Sun which forms such a marked feature in 
some of the photographs. There are also two segments of rings in a different part of 
the plate. 
The instrument used for measuring the distances apart of the images of the promi¬ 
nences was very, kindly lent to me by the Ptoyal ObservatoryGreenwdch. It was 
