200 
ences of illumination existing in different parts of a bright 
object, such as a sheet of drawing paper, which are so 
subtle as to be at the very limit of the power of recogni- 
tion of a trained eye, and even, as it appeared to me, 
those which surpass that limit. 
My first attempts at photographing the corona were 
made with photographic lenses, but uncertainty as to the 
state of correction of their chromatic aberration for this 
part of the spectrum, as well as some other probable 
sources of error which I wished to avoid, led me to make 
use of a reflecting telescope of the Newtonian form. The 
telescope is by Short, with speculum of 6 inches diameter, 
and about 3} feet focal length. Asmall photographic camera 
-was fastened on the side of the telescope tube, and the 
image of the sun after reflection by the small plane 
speculum was brought to focus on the ground glass. The 
absorptive media were placed immediately in front of the 
sensitive film, as in that position they would produce the 
least optical disturbance. Before the end of the telescope 
was fixed a shutter of adjustable rapidity which reduced 
the aperture to 2 inches. This was connected with the 
telescope tube by a short tube of black velvet for the pur- 
pose of preventing vibrations from the moving shutter 
reaching the telescope. On account of the shortness of 
the exposures it was not necessary to give motion to the 
telescope. 
It was now necessary to find an absorptive medium 
which would limit the light received by the plate to the 
portion of the spectrum from about Gto H. There is a 
violet (pot) glass made, which practically does this. I 
had a number of pieces of this glass ground and polished 
on the surfaces. Three or four of these could be used 
together, castor-oil being placed between the pieces to 
diminish the reflection of light at their surfaces. Some 
inconvenience was found from small imperfections within 
the glass, and it would be desirable in any future experi- 
ments to have a larger supply of this glass, from which 
more perfect pieces might be selected. 
In my later experiments I used a strong and newly 
made solution of potassic permanganate, in a glass cell 
with carefully polished sides. This may be considered 
as restricting the light to the desired range of wave-length, 
since light transmitted by this substance in the less 
refrangible parts of the spectrum does not affect the 
photographic plates. 
Different times of exposure were given, from so short 
an exposure that the sun itself was rightly exposed, to 
much more prolonged exposures, in which not only the 
sun itself was photographically reversed, but also the 
part of the plates extending for a little distance from the 
sun’s limb. 
Gelatine plates were used, which were backed with a 
solution of asphaltum in benzole. 
After some trials I satisfied myself that an appearance 
peculiarly coronal in its outline and character was to be 
seen in all the plates. I was, however, very desirous of 
trying some modifications of the methods described, with 
the hope of obtaining a photographic image of the corona 
of greater distinctness, in consequence of being in more 
marked contrast with the atmospheric illumination. 
Our climate is very unpropitious for such observations, 
as very few intervals, even of short duration, occur in 
which the atmospheric glare immediately about the sun 
is not very great. Under these circumstances I think it 
is advisable to describe the results I have obtained, with- 
out further delay. 
The investigation was commenced at the end of May 
of this year, and the photographs were obtained between 
June and September 28. 
The plates which were successful are twenty in number. 
In all these the coronal form appears to be present. This 
appearance does not consist simply of increased photo- 
graphic action immediately about the sun, but of distinct 
coronal forms and rays admitting in the best plates of 
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J ya se ee 
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2 = 7 
[Dec. 28, 1882 
measurement and drawing from them. This agreement 
in plates taken on different days with different absorptive 
media interposed, and with the sun in different parts of 
the field, together with other necessary precautions ob- 
served, makes it evident that we have not to do with any 
instrumental effect. 
The plates taken with very short exposures show the 
inner corona only, but its outline can be distinctly traced 
when the plates are examined under suitable illumination. 
When the exposure was increased, the inner corona is 
lost in the outer corona, which shows the distinctly curved 
rays and rifts peculiar to it. 
In the plates which were exposed for a longer time. not 
only the sun:but the corona also is photographically re- 
versed, and in these plates, having the appearance of a 
positive, the white reversed portion of the corona is more 
readily distinguished and followed in its irregularly sinu- 
ous outline than is the case in those plates where the sun 
omy, is reversed, and the corona appears, as in a negative, 
ark. 
Prof. Stokes was kind enough to allow me’to send the 
originals to Cambridge for his examination, and I have 
his permission to give the following words from a letter I 
received from him: “The appearance is certainly very 
corona-like, and I am disposed to think it probable that 
it is really due to the corona.” 
Prof. Stokes’ opinion was formed from the appearance 
on the plates alone, without any knowledge of their orien- 
tation, and without the means of comparing them with 
the eclipse plates taken on May 17. 
I have since been allowed, through the kindness of 
Capt. Abney, to compare my plates with those taken of 
the corona in Egypt during the eclipse of May last. 
Though the corona is undergoing doubtless continual 
changes, there is reason to believe that the main features 
would not have suffered much alteration between May 17 
and September 28, when the last of my plates was taken. 
This comparison seems to leave no doubt that the object 
photographed on my plate is the corona. The more pro- 
minent features of the outer corona correspond in form 
and general orientation, and the inner corona, which is 
more uniform in height and definite in outline, is also very 
similar in my plates to its appearance in those taken 
during the eclipse. 
Measures of the average height of the outer and of the 
inner corona in relation to the diameter of the sun’s 
image are the same in the eclipse plates as they are in 
my plates taken here. 
There remains little doubt that by the method described 
in this paper, under better conditions of climate, and 
especially at considerable elevations, the corona may be 
successfully photographed from day to day with a 
definiteness which would allow of the study of the 
changes which are doubtlessly always going on init. By 
an adjustment of the times of exposure, the inner or the 
outer corona could be obtained as might be desired. It 
may be that by a somewhat greater restriction of the 
range of refrangibility of the light which is allowed to 
reach the plate, a still better result may be obtained. 
Plates might be prepared sensitive to a limited range 
of light, but the rapid falling off of the coronal light 
about H would make it undesirable to endeavour to do 
without an absorptive screen. Lenses properly corrected 
might be employed, but my experience shows that 
excessive caution would have to be taken in respect of 
absolute cleanness of the surfaces and of some other 
points. There might be some advantage in intercepting 
the direct light of the sun itself by placing an opaque disk 
of the size of the sun’s image upon the front surface of 
the absorptive screen. Though for the reasons I have 
already stated I did not attempt eye-observations, there 
seems no reason why, with suitable screens and under 
suitable atmospheric conditions, the corona should not be 
studied directly by the eye. There might be some 
