TOTAL SOLAE ECLIPSE OF JULY 18, 1860. 
407 
regard to altitude, and in other respects. I have ascertained in this Tvay that the faculse 
occupy the highest positions of the sun’s photosphere, the spots appearing like holes 
in the penumbrse, which appeared lower than the brighter regions surrounding them ; 
in one case parts of the faculse were discovered to be sailing over a spot, apparently at 
some considerable height above it. 
My hope of rendering evident the luminous prominences is dependent upon an exten- 
sion of this experiment. I believe that, with a careful adjustment of the time of expo- 
sure of the sensitive plate, I shall succeed in obtaining the outline of the luminous pro- 
minences (the so-called red flames) as very delicate dark markings on the more brilliant 
mottled background of the photosphere. These delineations, except with the aid of 
the stereoscope, would be confounded with the other markings of the sun’s surface, but 
they would assume their true aspect, and stand out from the rest, as soon as two suit- 
able pictures were viewed by the aid of that instrument. 
The difficulties in the way of doing this are, however, of a special kind, as will 
readily be seen from the following considerations : when the aperture of the instan- 
taneous slide is at a maximum, and the rapidity of motion at a minimum, a picture of 
the sun will result which will be of one uniform maximum density, without the slightest 
trace of any marking even of a dark sun-spot. As the aperture is reduced and the velo- 
city of the slide augmented the spots will become depicted, but no trace of the penumbrse 
will be seen ; then we shall get the penumbrse, and subsequently traces of the faculas and 
of the general mottling of the sun’s disk ; lastly, by still further reducing the aperture, 
the faculse and the mottling will be well brought out, but especially the latter. The 
photographic process, it will be recollected, is one of progressive action, and even the 
faintest parts of the picture may, by a long exposure, produce as much intensity as the 
bright parts do by a shorter action ; and evidently, with sufficient time, all distinctions 
of bright and less bright must cease to exist on the photographic plate, if all parts have 
produced the maximum density of effect which the plate is capable of affording. It 
rarely (it might be said never) happens that all parts of the picture are portrayed with 
the best effect ; and in heliography the apparatus has to be variously adjusted, according 
as the spots or the mottlings of the sun’s surface are required to be especially well shown. 
Measurements of the Totality-Photographs. 
The main object of the observations of the total eclipse of 1860 was to ascertain 
whether the luminous prominences are objective phenomena belonging to the sun, or 
whether they are merely subsidiary phenomena, produced by some action of the moon’s 
edge on light emanating originally from the sun. If the luminous prominences are 
attached to the sun, it is evident that they would continually change their positions with 
respect to the moon’s centre as the moon moved across the solar disk. For example, a 
prominence situated in the direction of the moon’s path would maintain its position- 
angle in reference to the moon’s centre unchanged, but it would be gradually and at 
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