ABSTRACT
The realm of the extragalactic nebulae has been extensively investigated only during the past three or four decades and even at the beginning of the present century the true nature of these objects was a matter of conjecture and dispute. On the one hand, there were those astronomers who considered them to be mere satellites of our own system, lying at distances of only 20,000 or 30,000 light-years, and on the other were the proponents of the island-universe theory, who believed that these nebulae lay at tremendous distances and were comparable in size to our own. Sir William Huggins had already demonstrated the existence of two classes of nebulae by means of the spectroscope—those that were purely gaseous and clearly lay within the boundaries of the Galaxy, and others whose spectroscopic features indicated that they were composed of many thousands or millions of stars.
