With greater telescopic power additional features begin to reveal themselves; the mist immediately above the trapezium assumes a roughly triangular shape, and is evidently much denser than the rest of the nebula, presenting a curdled appearance similar to that of the stretches of small cloud in a 'mackerel' sky; while from the upper jaw of the fish-mouth a great shadowy horn rises and stretches upward, until it gradually loses itself in the darkness of the background. This wonderful nebula appears to have been discovered in 1618, but was first really described and sketched by Huygens in 1656, since when it has been kept under the closest scrutiny, innumerable drawings of it having been made and compared from time to time with the view of detecting any traces of change. The finest drawings extant are those of Sir John Herschel and Mr. Lassell, and the elaborate one made with the help of the Rosse 6-foot mirror. Drawing, however, at no time a satisfactory method of representing the shadowy and elusive forms of nebulæ, has now been entirely superseded by the work of the sensitive plate. Common, Roberts, Pickering, and others have succeeded admirably in photographing the Great Nebula with exposures ranging from half an hour up to six hours. The extension of nebulous matter revealed by these photographs is enormous (Plate XXX.), so much so that many of the central features of the nebula with which the eye is familiar are quite masked and overpowered in the photographic print. The spectrum of the Orion Nebula exhibits indications of the presence of hydrogen and helium, as well as the characteristic green ray which marks the unknown substance named 'nebulium.'