  We have imaged the z=3.594 radio galaxy 4C +19.71 in the light of the
redshifted [OIII] 5007 angstrom emission line, using a narrow-band filter
centered at 2.3 microns with the Near Infrared Camera on the Keck Telescope.
The [OIII] nebula of 4C +19.71 has a size of 74 x 9 kpc, and a luminosity of 3
x 10^37 W. The rest frame equivalent width of the 5007 line, averaged over the
entire nebula, is 560 angstroms. The length of the major axis of the [OIII]
emission is nearly identical to the separation of the radio lobes seen at 1465
MHz (Rottgering, et al. 1994), and the position angle of the nebula is the same
as that of the two radio lobes. In addition, 4C +19.71 follows the optical
emission line vs. radio power correlation seen in other powerful radio
galaxies. The [OIII] and Lyman alpha emission line luminosities suggest that
the ionized gas mass lies in the range of 2 x 10^8 - 10^9 solar masses. The O/H
ratio in the nebula is at least a few tenths solar, and may be as high as a
factor of three above solar, indicating a previous phase of star formation in
4C +19.71. Thirty five percent of the total K-band flux is contributed by the
5007 angstrom emission line, and the continuum of 4C +19.71 has a K=19.6 mag.
This places 4C +19.71 along the K-z relation found for other radio galaxies and
radio loud quasars. If the continuum is dominated by starlight, the host galaxy
has a rest frame visual luminosity of about 40L*. There are no candidate
emission-line objects at the redshift of 4C +19.71 having [OIII] rest frame
equivalent widths of more than about 2% that of the radio galaxy itself within
a volume of 212 cubic Mpc.
