  The 35-ton prototype for the Deep Underground Neutrino Experiment far
detector was a single-phase liquid argon time projection chamber with an
integrated photon detector system, all situated inside a membrane cryostat. The
detector took cosmic-ray data for six weeks during the period of February 1,
2016 to March 12, 2016. The performance of the photon detection system was
checked with these data. An installed photon detector was demonstrated to
measure the arrival times of cosmic-ray muons with a resolution better than 32
ns, limited by the timing of the trigger system. A measurement of the timing
resolution using closely-spaced calibration pulses yielded a resolution of 15
ns for pulses at a level of 6 photo-electrons. Scintillation light from
cosmic-ray muons was observed to be attenuated with increasing distance with a
characteristic length of $155 \pm 28$ cm.
