400 
ME, WAEEEN DE LA ETJE ON THE 
In order that the phenomena presented by the photographs may be clearly understood, 
T propose to give an explanation of certain appearances in them, which might other- 
wise occasion some difficulty. First, Avith regard to No. 25 photograph (the first totality- 
picture) (Plate IX. fig. 1). The sensitive plate was in the heliograph and the shde 
which covered it removed, a minute or so before totality, a temporary screen being first 
held just before the object-glass, so as to stop off all light. Thus evei^ thing was in 
readiness, and it was only necessary to remove the temporary screen, to expose the plate 
at the proper moment. The very instant of the disappearance of the sun, I gave the 
signal for its removal, which was immediately done, and Mr. Beckley, who was watching 
the chronometer, gave the signal for covering the object-glass exactly one minute after 
it had been uncovered. I had given instructions that no attempt was to be made to 
note the precise epoch of total obscuration ; for each operator had too much to occupy 
his attention to admit of any work being done which was not absolutely essential to 
the photographic operations. In order to regulate the time of exposure, the precise 
position of the second-hand of the chronometer Avas noticed when the plate was first 
exposed, and the signal Avas given for replacing the screen when the second-hand had 
completed a revolution. 
The telescope followed the motion of the sun so Avell that the prominences retained 
a perfectly fixed position on the sensitive plate; and from the results on the second 
plate, presently to be spoken of, it is knoAvn that they must have depicted themseh es 
to some extent, though very faintly, in a second. On the other hand, the comparatively 
feeble corona would have required even a longer period to thoroughly imprmt itself 
than the whole time allotted for the first picture. Consequently, as the moon moved 
from the Avest to the east, she kept shutting otf the prominences and corona on the 
east, thus stopping further action ; Avhile, on the west, she permitted fresh portions of 
the corona to commence a new action. The luminous prominences, when once they 
had produced their effect, could not be obliterated, although they might be subse- 
quently covered by the moon ; for it is well known in photography that latent images 
remain on the plate for a long time, and become apparent on applying the develop- 
ing fluid. In the case of the corona, the full effect not having been produced eA'eu at 
the end of the operation, it will be evident that its picture was necessarily the most 
intense on the eastern side, just at that part where the moon’s periphery had arriA'ed at 
the close of the work ; while it is clear that on the Avestern side the action Avould con- 
tinuously folloAV up the moon’s progress, and that therefore an impression gradually 
becoming fainter towards the lunar disk, Avould indicate the point Avhich the moon had 
reached when the image of the eclipse was shut off. With this explanation, tlie 
appearance of the moon’s edge beyond the prominences on the eastern side in the first 
totality-picture can present no difficulty. If, during the exposure of the plate, fresh 
prominences had become uncovered on the Avestern side, they Avould liaA'e imprinted 
themselves ; and if the plate had remained in the heliograph durmg the enthe period of 
totality, the Avhole of the prominences Avould necessarily have recorded themsclA’es on a 
