Top plates showing the focus adjustment and sample holder ("Bird-head")
Bottom plate showing the hole for the water drop (A). The other hole is for a lanyard. The bird's eye is for liquid samples.
Surface tension of the water drop creates a refracting spherical surface.
Holes for the water drop (top) and liquid sample. Notice that the water drop hole has two diameters. This creates a biconvex lens with two different diameter spherical surfaces. According to Gray such a lens yielded a sharper image.