A star cluster. Table III, Figure 25 of Ursin's Popular Introduction to Astronomy. "William Herschel divides stellar nebulae [stjörnuþokur] into categories and calls the first category 'star clusters' [stjörnuþyrpingar]." One of these star clusters is found "between the stars eta and zeta in the constellation Hercules and can be seen with the naked eye. Through a telescope of relatively low power it looks like a round patch of mist, similar to a comet. But when Herschel examined it through a telescope twenty feet in length, it had the appearance shown in Figure 25 (3E422)."
Source: G.F. Ursin, Populært Foredrag over Astronomien, Table III, Figure 25 [digitally edited].