So different did the prominences appear to different eyes, and so many were the theories as to what they were, that no general consensus of opinion was reached, and some thought them no part of either sun or moon, but a mere mirage or optical illusion. But at the return of this eclipse in 1860, photography was employed so as to demonstrate beyond a shadow of doubt the real existence and true solar character of the prominences. By the slow progress of the moon across the sun and the prominences on the edge, a unique series of photographs by De la Rue showed the moon's edge gradually cutting off the prominences piecemeal on one side of the sun, and equally gradually uncovering them on the opposite side. The prominences, then, were known to be real phenomena of the sun, some of them disconnectedly floating in his atmosphere, as if clouds. Their forms did not vary rapidly, they were very abundant, and their light was so rich in rays of great photographic intensity that many were caught on the plate which the eye failed to see; they appeared at every part of the sun's limb and their height above it indicated that they must be many thousand miles in actual dimension. What they were, however, remained an entire mystery, and no one even thought it possible to find out what their chemical constitution might be or to measure the speed with which they moved. A few years later came the great Indian eclipse (August 28, 1868), at that date the longest total eclipse ever observed. Janssen of France and many others went out to India to witness it. Fortunately the prominences were very brilliant and this led Janssen to believe it would be possible for him to see them the day after the eclipse was over. By modifying the adjustment of his apparatus suitably and changing its relation to the sun's edge, he found that hydrogen is the main constituent in the light of the prominences. In addition to this he was able to trace out the shapes of the prominences, and even measure their dimensions. His station in India was at Guntoor, many weeks by post from home; so that his account of this important discovery reached the Paris Academy of Sciences for communication with another from the late Sir Norman Lockyer of England, announcing a like discovery, wholly independently.