In due time, one astronomer after another began to admit that possibly the remarkable form which Lord Rosse announced as characteristic of some nebulæ might not be a mere figment of the imagination. The complete vindication of Lord Rosse’s great discovery was not, however, attained until that wonderful advance in the arts of astronomy when the photographic plate was called in to supplement, or rather vastly to extend, the powers of the eye. Dr. Isaac Roberts not only showed by a magnificent photograph that the Great Spiral discovered by Lord Rosse was just as Lord Rosse had described it, he not only showed that the other spirals announced by Lord Rosse were equally entitled to the name, but, with the newly acquired powers that the photographic plate placed at his disposal, he was able to show that many other nebulæ, which had been frequently observed and had even been sketched, possessed further features too faint and delicate to be seen by any human eye, even with the help of the most powerful telescope. These further features were discovered because they came within the ken of the intensely acute perception of the photographic plate. On the plate these features which the camera showed, were added to those which the eye had already perceived, and when these additions were made it was not infrequently found that the nebula assumed the form of a spiral. But the most remarkable circumstance has still to be added. Some of the plates exposed by Dr. Roberts show clear and unmistakable photographs of spiral nebulæ as exquisite in detail as the Great Spiral itself, but yet so faint that they have never been seen by the eye in any telescope whatever, though they could not elude the photographic plate. Thus, Dr. Roberts not only confirmed in the most splendid manner that really great discovery of the spiral nebulæ of which the honour belongs to Lord Rosse, but the eminent photographic astronomer added many other spirals of the greatest interest to the list of those objects which Lord Rosse had himself given. Though these discoveries placed the fact of the existence of spiral nebulæ in an impregnable position, and though they greatly increased the interest with which astronomers study such objects, yet another stop had to be taken before the spiral nebula attained the position of extraordinary importance as a celestial object which must now be acknowledged to be its due. [Illustration: Fig. 30.—A GROUP OF NEBULÆ (_Lord Rosse_). (3440, 3445 in n.g.c.) (_From the Scientific Transactions of the Royal Dublin Society._)]