ON THE TOTAL SOLAR ECLIPSE OP AUGUST 29, 1886. 
307 
being at the further end. The beam enters this tube at one side of the centre, and is 
reflected back at a small angle from the mirror at the further end to the photogi’aphic 
plate, which is alongside of the place where the light enters. A partition in this tube 
separates the incoming and reflected rays for as long a distance as possible. Below 
this partition there are oval diaphragms, through which both beams pass. I may 
here remark that this last precaution is, I rather think, a mistake ; for the backs of 
most of these diaphragms are necessarily visible from the photographic plate, and, to 
whatever extent they are useful in preventing stray light from getting to the bottom 
of the tube, they are proportionately harmful in reflecting this light directly back on 
to the photographic plate, which is very near to them. I was so much afraid of the 
direct Sun light striking the back of these oval diaphragms that I placed a temporary 
diaphragm of only 2 inches in diameter at the bottom of the outer tube, thus ensuring 
that the beam of Sun light should pass clear through the diaphragms in the inner 
tube. I should gladly have dispensed with this if I had dared. 
The two tubes are held in position in a long wooden frame. There is a half-open 
space between the tubes, which has to be covered up by a cloth or other temporary 
contrivance. This opening should be provided with a permanent cover. 
I arranged an instantaneous shutter close in front of the plate, that is to say, in 
the position which Dr. Huggins considered most advantageous. It consisted of a long 
wooden slide, with a rectangular opening 6 inches wide, which was drawn across an 
opening of 2| inches diameter by two pieces of elastic. It was released by cutting a 
thread. Both the elastics and the thread were fastened to the body of the telescope, 
and not to the camera or slide which takes the plate. This latter part is not too 
firmly fixed to the body, and I was anxious to avoid vibrations. 
As Dr. Schuster has fully described the adjustment of his instrument for equatorial 
movement, focus, &c., I will only remark that the same methods were used in the 
case of the coronagraph in so far as they were applicable. 
2. Observations in Sun Light and during the Partial Eclipse. 
The most important observations to be made with the coronagraph were with the 
view of testing the practicability of obtaining photographs of the corona during 
Sun light by this instrument. This could be done in two ways :— 
(1.) By obtaining photographs shortly before or after the eclipse, and comparing 
any irregularity that might appear in the halo round the Sun with photographs of the 
corona taken during totality—a similarity of form indicating that the corona had been 
photographed. 
(2.) By taking photographs during the partial eclipse. Then, if the light of the 
corona produced any effect on the plate, the dark limb of the Moon would be seen 
against it. 
It was advisable in trying the first of the above experiments to take a series of 
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