TOTAL SOLAE ECLIPSE OE JULY 18, 1860. 
405 
when the distance between the moon and the sun’s peripheries was only 9"’6. This pho- 
tograph was obtained with the full aperture (3-4 inches) of the heliograph, and the plate 
was exposed by remo\dng a temporary cover which had been placed before the object- 
glass, and replacing it as quickly as possible. The time of exposure would certainly not 
exceed a second, yet the image is completely solarized (bleached) from over-exposure; 
moreover, the wind, which rose suddenly at that period, violently shook the heliograph 
in the direction of right ascension, by successive impulsions against the object-end, which 
projected beyond the walls of the observatory ; and many impressions of the solar crescent 
are consequently depicted on the plate, on which, however, not the shghtest trace of 
prominence A could be made out. With the aperture of the object-glass reduced to 
2 inches in diameter, and using the instantaneous apparatus, a picture of partial phase 
could under similar circumstances have been procured in -^th of a second, and therefore 
in less than -^th of a second with the full aperture of the telescope. Moreover, as in 
the second totality-picture (No. 26) the prominences were depicted three times, in con- 
sequence of two disturbances of the telescope in right ascension, during the minute the 
plate was exposed in the heliograph, we know that on the average twenty seconds are 
about sufficient to bring out the picture of the luminous prominences strongly. These 
triplicate images are not, however, of equal intensity, one being very faint ; and there- 
fore, assigning to this latter (what its appearance warrants) an exposure of half the time 
of the other two, we have twelve seconds as the time required to depict the most lumi- 
nous of the prominences fairly. It results, therefore, that the light of the luminous 
prominences is fully 58x12=696 times less bright than that of the photosphere of the 
sun. 
On August 12, 1862, 1 succeeded, as I have already stated in the foot-note, page 334, 
in obtaining an extremely faint impression of the moon with the Kew heliograph in three 
minutes. The full aperture of the object-glass was employed, and the chemicals used 
were in the highest degree of sensitiveness. An impression of the luminous prominences of 
equal intensity would, according to data furnished by the second totality-picture No. 26, 
have been produced in a second. It may therefore be safely estimated that the image of 
the luminous prominences (for equal areas) is 180 times more brilliant than that of 
the moon. Assuming for the ratio of the light of the sun in comparison with the light of 
the moon 200,000 to 1, it would follow that the image of the luminous prominences is 
■—^^^=1111 times less brilliant than that of the sun; taking the mean of the two 
estimates it would be 900 times less brilHant than that of the sun. , , 
Although in all probability the prominences are less bright than the dark nuclei of 
the solar spots, it does not follow that they would appear as very dark markings on the 
sun’s disk, for to a great extent they may permit of the transmission of the light emitted 
by the photosphere ; and, besides, it is by no means probable that there is any intimate 
connexion between the solar spots and the prominences, for the vast extent of the sun’s 
limb which is surrounded by the prominences precludes such an idea, and leads to the 
MDCCCLXII. 3 I 
