seen corresponded with the brightest line of the three; and when a second line was visible, this also was no new line, but agreed with the second brightest line in the three-line spectrum. The fourth line was seen only in the spectrum of a very bright, small, blue planetary nebula, but was later observed in other cases, and especially in the great Orion nebula." At this time the latter was not visible, but when Dr. Huggins had opportunity to examine it, he says, "The telescopic observations of this nebula seem to show that it is suitable to a crucial test of the usually received opinion that the resolution of a nebula into bright stellar points is a certain indication that the nebula consists of discrete stars." Professor Proctor says, "A simple glance resolved the difficulty. The light from the brightest part of the nebula--the very part which under Lord Rosse's great reflector blazed with innumerable points of light--gave a spectrum identical in all respects with that which Huggins had obtained from the planetary nebulæ. Thus, what had been deemed boldness in Herschel--namely, that he should have associated the wildest and most fantastic nebula in the heavens with the circular and (in ordinary telescopes) almost uniformly luminous planetary nebulæ--was unexpectedly confirmed." The spectrum of this nebula has more recently been photographed by a long exposure in the camera of the prepared plate. Of the result, Professor Proctor thus speaks, "The nebula is seen to be in great part gaseous, and, where gaseous, to shine in the main with the tints described above; but parts of the nebula are not gaseous, and those portions which are so are not all constituted in the same manner.... That portion which is called the fish's mouth gives a continuous spectrum; in other words, the same spectrum which we obtain from a star or a star-cluster. This is the spectrum arising from a glowing solid or liquid mass, or if from a gaseous body, then the gaseous body must be in a state of great compression.... But the stars thus forming must be immersed in the glowing gas forming the general substance of the nebula.... It would be absurd to suppose that the nebula is a flat surface; ... nebulous matter lies also, in all probability (certainly one might fairly say), between us and the stellar aggregration as well as on the farther side." Further, the same author says, "If, as is probable, the luminosity of the gaseous portion of the Orion nebula is accompanied by but a relatively small proportion of heat, then the rays from the violet and ultra-violet part of the spectrum are likely to give us much more complete information respecting the constitution of these nebulous masses than can be derived from the visible part of the spectrum."