Also at the base of the rhomboid, between β and γ, one-third of the way from β, a small telescope will disclose a nebula which has assumed the shape of a ring, or at least it looks like a ring at this distance. There are various types of nebulæ, some, like the "planetary" and "spiral," having definite forms, others being as shapeless as a puff of vapor. Sometimes a planetary nebula has a star at its center, and again it appears hollow, like "a little smoke ring." It is then called a ring nebula. The most famous of these is the one found in Lyra. On the 19th and 20th of April swift meteors, known as the Lyrids, radiate from the vicinity of this constellation, although the display is of interest rather than of any particular beauty. The legend of this celestial harp, which is often hung by map artists around the neck of an eagle, is one of the most popular stories in mythology, and its very appearance in the heavens brings to mind the beautiful lovers, Orpheus and Eurydice. The harp was anciently represented as having been invented by Mercury, who gave it to his half-brother Apollo, the Sun-god, who later presented it to Orpheus, the son of a Muse. Mercury, as an infant, gave promise of being a most remarkable god, for the very day that he was born, he climbed out of his cradle, wandered out of the lofty cavern of Cyllene and picked up a little tortoise that was crawling past the entrance. Gently pulling off the scoop-shaped shell, he bored neat holes along its edges, stretched dried tendons across its face, and wedged it firmly between two horns. His tiny hands then strayed across the strings, and he danced delightedly as the most delicious music issued from the shell. "And this, whilst yet Encradled, Hermes pierced and called it Lyre." --_Aratus._ Mercury then went in and slept awhile, but when darkness came, he again slipped out of his cavern and ran over to the hillside to see what further adventure awaited him. Here he found the pasture of Apollo's cows, and looking the animals over, separated fifty of the best from the herd, wrapped their hoofs in branches and drove them backwards into a large cave where he made a fire and sacrificed two of them to the twelve Gods (himself included). [Figure 68: THE RING NEBULA IN LYRA. Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope.]