How did William Herschel use star counts to study the Milky Way?

William Herschel used star counts, which he termed "star gages," to deduce the structure of the Milky Way and create the first quantitative chart of its form in 1785.12

His method relied on two primary, though later abandoned, assumptions:
* That his telescope was powerful enough to see all the stars within our system.2
* That stars were spread out regularly throughout the system.2

Herschel reasoned that if he aimed his telescope toward the plane of the Milky Way, the number of stars visible in the field of view would serve as a measure of the distance from the Sun to the edge of the galaxy in that specific direction. By performing a large number of these counts, he concluded that the Milky Way was a flattened disk. While he eventually realized his telescope could not actually penetrate to the galaxy's edge, his work remains a landmark example of the early use of stellar statistics to interpret the spatial extent of the Galaxy.12