404 
ME. WAEEEN DE LA EIJB ON THE 
appears to be much greater than that portion of the moon’s limb corresponding in 
position-angle with it. 
On comparing the results of the expedition of 1860 with those obtained in 1851, one 
cannot fail to be impressed with the general similarity in the aspect of the prominences 
at the two epochs : on both occasions were seen luminous masses of vast extent, per- 
fectly detached from the sun, and far beyond the lunar disk ; the same iiTegularity of 
outline on the convex side, running out into points; the same apparent outpom-ing 
of faint vapours, falling as it were towards the sun (as in the faint portions of pro- 
minence A) ; lastly, although not seen with the eye in 1860, there is recorded by the 
photographic retina a similar hooked projection to that seen in 1851, and named by the 
Astronomer Eoyal “ the boomerang,” from its approximation in form to the Austrahan 
weapon bearing that name. 
Reference to Plate XV. affords better means of judging of the dimensions of the 
protuberances depicted on the two photographs than the photographs themselves, on 
account of the latter merely showing their distance from the moon’s periphery, whereas 
in the index map, Plate XV., the distances from the sun’s disk are shown. We thus 
see that while the two photographs give a fah representation of the height above the 
sun’s periphery of the prominences D, E, F, G, M, N, O, P, Q, R, they do not do so in 
respect of the remainder. Notably, the prominence A, if it extended inwards to the 
sun’s periphery, and was not, like the floating cloud D, supported at some distance from 
it, must have been of twice the height revealed to us ; and again, the prominence K 
must, as regards the brighter part of it, have been nearly four times the height seen 
above the moon’s limb. The brighter part of the prominence K, at the epoch of the 
second totality-picture, was covered by the moon; and although the fainter hooked part 
projected beyond the lunar disk, it was too faint to imprint itself in the second picture, 
on account of the disturbance of the telescope, which did not allow it to remain suffi- 
ciently long on one part of the plate. In the first picture it depicted itself clearly, and, 
if it had done so in the second, would have admitted, in connexion with A, of a chord 
being drawn which would have afforded a capital basis for measurements. The actinic 
power of “ the boomerang ” is quite remarkable ; for it imprinted itself, distinctly, three 
times on photograph 26, although it was invisible to the eye. Not only, therefore, is 
photography of value in recording phenomena visible to the human eye, but it is also 
able to render evident bodies which emit only those rays which belong to the invisible 
part of the spectrum. 
The question whether the luminous prominences would appear as bright or dark 
markings on the sun’s disk admits of a probable solution by means of photography, 
which furnishes data as to the degree of luminosity of the prominences relatively to that 
of the sun’s photosphere. It will be recollected that I have stated that one of the pro- 
minences (A) was visible some minutes previous to totality, and that it continued visible 
to other observers several minutes after the reappearance of the sun. It so happened that 
photograph No. 28 was taken about twenty seconds after the reappearance of the sun. 
