lem which confronted Herschel was two-fold: (1) the scale, and (2) the structure of the stellar system. In 1781 he had written a paper on the parallaxes of the fixed stars, but his investigation only yielded a negative result. Accordingly he concentrated on the question of the structure of the universe and the arrangement of its component parts.
In 1783 the publication of Messier' s first catalogue directed Herschel's attention to the star-clusters and nebulae. As soon as it came into his hands, he says, he applied his 20-feet reflector to the nebulae, "and saw with the greatest pleasure that most of the nebulae which I had an opportunity of examining in proper situations yielded to the force of my light and power and were resolved into stars". Accordingly, Herschel decided to "sweep" the heavens with two main objects in view: (1) to search systematically for new nebulae, and (2) to gauge the extent of the sidereal system by counting the number of stars visible in different regions of the heavens. In 1784, in his preliminary paper on the construction of the heavens, he described his method of star-gauging, which, he said, "consists in repeatedly taking the number of stars in ten fields of view of my reflector very near each other, and by adding their sums and cutting off one decimal on the right, a mean of the contents of the heavens, in all the parts which are thus gauged, is obtained".
In his paper on "The Construction of the Heavens," dated 1st January, 1785, Herschel gave the results of his preliminary investigations and outlined his theory of the stellar system. "That the Milky Way," he said, "is a most extensive stratum of stars of various sizes admits no longer of the least doubt, and that our Sun is actually one of the heavenly bodies belonging to it is evident. I have now viewed and gauged this shining zone in almost every direction, and find it composed of stars whose number, by the account of these gauges, constantly
