Planetary nebulæ are so called because with ordinary powers they present somewhat of the appearance of a planet seen very dimly and considerably out of focus. The appearance of uniformity in their boundaries vanishes under higher telescopic power, and they appear to be generally decidedly elliptical; they yield a gaseous spectrum with strong evidence of the presence of 'nebulium,' the unknown substance which gives evidence of its presence in the spectrum of every true nebula, and has, so far (with one doubtful exception) been found nowhere else. The chief example of the class is that body in Draco which first yielded to Huggins the secret of the gaseous nature of the nebulæ. It lies nearly half-way between Polaris and Gamma Draconis, and is described by Webb as a 'very luminous disc, much like a considerable star out of focus.' It is by no means a striking object, but has its own interest as the first witness to the true nature of that great class of heavenly bodies to which it belongs. The multitude of nebulous bodies scattered over the heavens may be judged from the fact that Professor Keeler, after partial surveys carried out by means of photography with the Crossley reflector, came to the conclusion that the number within the reach of that instrument (36-inch aperture) might be put down at not less than 120,000. It is a curious fact that the grouping of this great multitude seems to be fundamentally different from that of the stars. Where stars are densely scattered, nebulæ are comparatively scarce; where nebulæ abound, the stars are less thickly sown. So much is this the case, that, when Herschel in his historic 'sweeps' of the heavens came across a notably starless region, he used to call out to his assistant to 'prepare for nebulæ.' The idea of a physical connection between the two classes of bodies is thus underlined in a manner which, as Herbert Spencer saw so early as 1854, is quite unmistakable. There remain one or two questions of which the very shortest notice must suffice--not because they are unimportant, but because their importance is such that any attempt at adequate discussion of them is impossible in our limited space. One of these inevitably rises to the mind in presence of the myriads of the heavenly host--the familiar question which was so pleasingly suggested to our growing minds by the nursery rhyme of our childhood. To the question, What is a star? it has now become possible to give an answer which is satisfactory so far as it goes, though it is in a very rudimentary stage as regards details.