What distinction did William Herschel begin making regarding nebulae classifications?

Answer

He divided them into nebulous (diffuse) and resolvable (suspected star clusters).

Herschel started charting the sky systematically and classified nebulae based on their visual appearance, distinguishing between diffuse clouds and those objects whose stars he could begin to resolve.

What distinction did William Herschel begin making regarding nebulae classifications?
discoveryAstronomernebulacelestial object