308 
CAPTAIN L. DARWIN, DR. A. SCHUSTER, AHD MR. E. W. MAUNDER 
pliotographs/in each of the reversed positions of the instrument. Then any irregu¬ 
larity in the illumination round the Sun which was reversed on turning the instrument 
over would clearly be proved to be due to instrumental causes ; whereas any irregu¬ 
larities which were visible in photographs taken in both positions, and which were not 
thus reversed, must be due to some outside causes, either atmospheric or coronal. In 
my instrument reversal was only possible near noon ; and on this account, as well as 
on account of the Sun being at its greatest elevation, the best results would be 
obtained by taking one series of photographs shortl}^ before noon in one position, and 
another series in the other position soon afterwards. 
With regard to the length of exposure, I thought that there was no advantage to 
be gained for either test by giving a longer exposure than was necessary for the air 
glare to produce a result on the plate. I estimate the exposure, that I gave with the 
automatic shutter at between one-tifth and one-tenth of a second. 
The plates which I used for these tests consisted of gelatino-chloride dry plates on 
paper and on glass. These were prepared for me by two makers : by Mr. A. Cowan 
(for Messrs. Marion and Co.), to whom I am indebted for making for me a specially 
prepared batch of plates, and by Mr. Warnerke. 
Chloride plates were chosen in preference to bromide plates, because it was considered 
that their relatively greater sensitiveness to ultra-violet rays would be advantageous 
ior distinguishing between the corona and the air glare. The plate to be chosen is no 
doubt the one which is most sensitive in that 23art of the spectrum whei’e there is 
the maximum difference of intensity of light in the spectra of the corona and of the 
sky. I may here remark that Dr. Schuster informs me that, according to his photo- 
gTaj:)h, the maximum difference would af^pear to be in the less refrangible part of the 
spectrum, rather than in the violet; and, if that is the case, bromo-iodide plates would. 
I think, have been preferable.'" Paper was considered better than glass, as tending to 
reduce the halation to a minimum ; as a fact, I relied chiefly on the paper negatives. 
I used various developers for these chloride plates, without observing any marked 
difference in the results:— 
(L) llydrokinone, 1 grain to the ounce. To 2 oz. of the above add ^ dr. of a 
saturated solution of jDotassic carbonate, with common salt as a restrainer. 
(2.) Potassic citrate, 136 grs. ; potassic oxalate, 44 grs. ; water, 1 oz. To 3 parts 
of the above add 1 part of ferrous sul])hate, 140 grs. ; sulphuric acid, 1 drop; water, 
1 oz. Potassic bromide was used as a restrainer. 
(3.) A weak ferrous sulphate developer. 
Some of the plates were developed in the West Indies, and some on my return 
home. The results aj^peared to be the same, but they were, I think, much easier to 
develop in England. Ice was used freely in Grenada, but I do not think the gelatine 
became quite hardened during the time of develoj^ment, and the jilates appeared to be 
more liable to fog. 
* See p. 341. 
