28 W. Huggins—Photographing the Solar Corona 
as possible the light which enters the telescope to that which 
comes from the sun and the sky immediately around him. 
The telescope tube aa 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 / falls upon the cen- 
tral 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 tothesun. The exquisite definition of the photographic 
images of the sun shows, as was to be expected, that the smal 
deviation from the axial direction—two inches in seven feet— 
does not affect sensibly the performance of the mirror. The 
whole apparatus is firmly strapped on to the ro of the 
equatorial, and carried with it by the clock moti 
The performance of the apparatus is very satiatootoee The 
photographs show the ser ’s image sharply defined; even small 
spots are seen. When the sky is free from clouds, but pre- 
senting a whity spend from the large amount of scattered 
light, the sun’s image is well defined upon an uniform back- 
ground of illuminated sky, without any great increase of illu- — 
mination immediately about it. It is only when the sky be- 
comes clear and blue in color that sureinnl appearances present 
themselves with more or less distinctness. 
n my earlier work with this apparatus I used cells contain- 
ing potassic permanganate in solution, which were placed close 
to the sensitive surface, and between it and the shutter. I 
was much troubled Lc the rapid decom position of the potassic 
permanganate under the influence of the sun’s light. When 
apparently clear to the eye, a lens revealed minute particles 
which Srosiiniok: themselves upon the glass plates of the cell, 
and gave an appearance of structure to any coronal appearance 
which was in the plate; besides, any diminution of the trans- 
parency of the solution by the presence of minute particles 
would produce scattered light on the plate. 
' I then tried a solution of iodine in carbon disulphide, but the 
same inconvenience presented itself. Very soon, under the 
sun’s light, the solution was found by examination with a lens 
to show signs of commencing decomposition. 
ven when the solution was sensibly clear there was some 
disadvantage from the unavoidable imperfection of polish of. 
the surface of the plates, which reveals itself under the condi- 
tions of strong light in which they are placed.. If, however, 
the violet (pot) class which I used at first could be obtained 
annealed and free from the imperfections usually present in it, 
it would serve most usefully as a selective screen. 
For these reasons, after some months’ work I decided to give 
