Another of the most beautiful, on account of its regularity, is that of the Centaur (Fig. 22). These groups often assume the most extraordinary shapes in the telescope, such as crowns, fishes, crabs, open mouths, birds with outspread wings, etc. We must also note the _gaseous nebulæ_, universes in the making, _e.g._, the famous Nebula in Orion, of which we obtained some notion a while ago in connection with its sextuple star: and also that in Andromeda (Fig. 23). [Illustration: FIG. 23.--The Nebula in Andromeda.] [Illustration: FIG. 24.--Nebula in the Greyhounds.] Perhaps the most marvelous of all is that of the Greyhounds, which evolves in gigantic spirals round a dazzling focus, and then loses itself far off in the recesses of space. Fig. 24 gives a picture of it. [Illustration: FIG. 25.--The Pleiades.] Without going thus far, and penetrating into telescopic depths, my readers can get some notion of these star-clusters with the help of a small telescope or opera-glasses, or even with the unaided eye, by looking at the beautiful group of the Pleiades, already familiar to us on another page, and using it as a test of vision. The little map subjoined (Fig. 25) will be an assistance in recognizing them, and in estimating their magnitudes, which are in the following order: Alcyone 3.0. Electra 4.5. Atlas 4.6. Maia 5.0. Merope 5.5. Taygeta 5.8. Pleione 6.3. Celæno 6.5. Asterope 6.8. Good eyes distinguish the first six, sharp sight detects the three others. In the times of the ancient Greeks, seven were accounted of equal brilliancy, and the poets related that the seventh star had fled at the time of the Trojan War. Ovid adds that she was mortified at not being embraced by a god, as were her six sisters. It is probable that only the best sight could then distinguish Pleione, as in our own day. The angular distance from Atlas to Pleione is 5'. The length of this republic, from Atlas and Pleione to Celæno, is 4'/23" of time, or 1°6' of arc; the breadth, from Merope to Asterope, is 36'.[8] In the quadrilateral, the length from Alcyone to Electra is 36', and the breadth from Merope to Maia 25'. To us it appears as though, if the Full Moon were placed in front of this group of nine stars, she would cover it entirely, for to the naked eye she appears much larger than all the Pleiades together. But this is not so. She only measures 31', less than half the distance from Atlas to Celæno; she is hardly broader than the distance from Alcyone to Atlas, and could pass between Merope and Taygeta without touching either of these stars. This is a perennial and very curious optical illusion. When the Moon passes in front of the Pleiades, and occults them successively, it is hard to believe one's eyes. The fact occurred, _e.g._, on July 23, 1897, during a fine occultation observed at the author's laboratory of Juvisy (Fig. 26).