of this object, both as to its actual physical nature and as to the materials present in it, is at present undetermined. This consideration lends a certain amount of mystery to Dr. Roberts' great photograph, a mystery we do not feel to a corresponding extent when we look at the photograph of Andromeda's only rival, the great nebula in Orion. The pictures of the latter exhibit a glorious object which is certainly known to be gaseous, and we have also the assurance that hydrogen is among the materials of which it is composed.
No part of Sir William Huggins' address was better than that which treated of the exquisite application of the spectroscope to the discovery of the movement of approach or movement of recession in the object from which the light emanates. In fact there is no passage in the address which seems to me more pregnant in significance than that in which he remarks that: "In the future a higher value may indeed be placed upon this indirect use of the spectroscope than upon its chemical revelations/' As to the accuracy of this method, it enables us, under favourable circumstances, to measure speed of recession or approach "to within a mile per second, or even less." What this means is that such a speed as that of the revolution of the earth in its orbit around the sun could be determined to within five or six per cent, of its amount. It is to Sir W. Huggins himself that we are indebted for the first application of this principle to astronomical measurement. The earliest observations were made by him in 1868, but for many years the application of this method was retarded by a want of perfection in the instruments necessary for so delicate a branch of research. However, such improvements have
