ON PHOTOGRAPHING THE SOLAR CORONA. 347 
inches in diameter, possesses great perfection of figure, and still retains 
its original fine polish. I decided not to use more than 3} inches of the 
central portion of the speculum, partly for the reason that a larger 
amount of light would be difficult of management, and partly because 
this restriction of the aperture would enable me to adopt the arrange- 
ment which is shown in the diagram. 
It will be seen at once from an inspection of the diagram that in this 
arrangement the disadvantage of a second reflection by the small mirror 
is avoided, as is also the mechanical inconvenience of tilting the speculum 
within in the tube as in the ordinary form of the Herschelian telescope. 
The speculum } remains in its place at the end of the tube a,d. The small 
plane speculum and the arm carrying it were removed. The open end of 
the tube is fitted with a mahogany cover. In this cover at one side is a 
circular hole f, 3’ diameter, for the light to enter ; below is a similar hole 
over which is fitted a framework to receive the ‘backs’ containing the 
photographic plates, and also to receive a frame with fine ground-glass 
for putting the apparatus into position. Immediately below, towards 
the speculum, is fixed a shutter with an opening of adjustable width, 
which can be made to pass across more or less rapidly by the use of 
india-rubber bands of different degrees of strength. In front of the 
opening / is fixed a tube c, six feet long, fitted with diaphragms, to restrict 
as far as possible the light which enters the telescope to that which 
comes from the sun and the sky immediately around it. The telescope- 
tube a, a, is also fitted with diaphragms, which are not shown in the 
diagram, to keep from the plate all light, except that coming directly from 
the speculum. It is obvious that, when the sun’s light entering the tube 
at f falls upon the central part of the speculum, the image of the sun will 
be formed in the middle of the second opening at d, about two inches 
from the position it would take if the tube were directed axially to the 
sun. The exquisite definition of the photographic images of the sun 
shows, as was to be expected, that this small deviation from the axial 
direction, two inches in seven feet, does not affect sensibly the perform- 
ance of the mirror. The whole apparatus is firmly strapped on to the 
refractor of the equatorial, and carried with it by the clock motion. 
The performance of the apparatus is very satisfactory. The photo- 
graphs show the sun’s image sharply defined ; even small spots are seen. 
When the sky is free from clouds, but presents a whity appearance from 
the large amount of scattered light, the sun’s image is well-defined upon 
a uniform background of illuminated sky, without any great increase of 
illumination immediately about it. It is only when the sky becomes clear 
and blue in colour that coronal appearances present themselves with more 
or less distinctness. 
In my earlier work with this apparatus I used cells containing 
