  Bubbles, the semi-circular voids below quiescent prominences (filaments),
have been extensively investigated in the past decade. However, hitherto the
magnetic nature of bubbles cannot be verified due to the lack of on-disk
photospheric magnetic field observations. Here for the first time, we find and
investigate an on-disk prominence bubble around a filament barb on 2019 March
18 based on stereoscopic observations from NVST, SDO, and STEREO-A. In
high-resolution NVST H$\alpha$ images, this bubble has a sharp arch-like
boundary and a projected width of $\thicksim$26 Mm. Combining SDO and STEREO-A
images, we further reconstruct 3D structure of the bubble boundary, whose
maximum height is $\thicksim$15.6 Mm. The squashing factor Q map deduced from
extrapolated 3D magnetic fields around the bubble depicts a distinct
arch-shaped interface with a height of $\thicksim$11 Mm, which agrees well with
the reconstructed 3D structure of the observed bubble boundary. Under the
interface lies a set of magnetic loops, which is rooted on a surrounding
photospheric magnetic patch. To be more persuasive, another on-disk bubble on
2019 June 10 is presented as a supplement. According to these results obtained
from on-disk bubble observations, we suggest that the bubble boundary
corresponds to the interface between the prominence dips (barb) and the
underlying magnetic loops rooted nearby. It is thus reasonable to speculate
that the bubble can form around a filament barb below which there is a
photospheric magnetic patch.
