When we reach the irresolvable nebulæ, we unquestionably have approached the creative period of solar systems and in many cases of whole galaxies. These are multifarious in form, but all can be reduced to a few comprehensive types. In determining the question as to whether these irresolvable nebulæ were composed of distinct stars like the Milky Way, but too distant to be resolved from their mist-like light into discrete stars by the most powerful telescopes, or whether they were gaseous in constitution,--that is, composed of diffused gaseous elements not condensed into solar bodies,--the spectroscope became the final and infallible test. Of this instrument, thus used, Professor Proctor, in his "Star-Clouds and Star-Mist," says, "A very few words will explain the whole matter to readers who remember the three fundamental laws of this new mode of investigation,--viz., that, first, light from a burning solid or liquid source gives the rainbow-colored streak of light commonly known as the prismatic spectrum; secondly, when vapors surround such a source of light, the rainbow-colored streak is crossed by dark lines; and, thirdly, when the source of light is gas, there is no longer a rainbow-colored streak, but merely a finite number of bright lines." Dr. Huggins selected for investigation the small planetary nebula in the Dragon. He says, "When I had directed the telescope armed with the spectrum apparatus to this nebula, I at first suspected that some derangement of the instrument had taken place, for no spectrum was seen, but only a short line of light. I then found that the light of this nebula, unlike any other extra-terrestrial light which had yet been subjected by me to prismatic analysis, was of definite colors, and therefore could not form a spectrum. A great part of the light is monochromatic, and so remains concentrated in a bright line occupying a position in the spectrum corresponding to its color. Careful examination showed a narrower and much fainter line near the one first discovered. Beyond this point, about three times as far from the first line, was a third exceedingly faint line. From the position of one of the bright lines it is inferred the gas nitrogen is one of the constituents of the nebula; another line indicates the existence of the gas hydrogen in that far-off system; the third line has not yet been associated with any known terrestrial element, though it is near one belonging to the metal barium, and still nearer one belonging to oxygen; a fourth line occasionally seen belongs to hydrogen." Professor Proctor says, "Dr. Huggins examined a large number of the planetary nebulæ (so called), obtaining in each case a spectrum which indicates gaseity. In some cases only one line could be seen, in others two, more commonly three, and in a few instances four. When these lines were seen they invariably corresponded in position with those already described. The single line sometimes