  We present the optical transmission spectrum of the hot Jupiter WASP-104b
based on one transit observed by the blue and red channels of the DBSP
spectrograph at the Palomar 200-inch telescope and 14 transits observed by the
MuSCAT2 four-channel imager at the 1.52 m Telescopio Carlos Sanchez. We also
analyse 45 additional K2 transits, after correcting for the flux contamination
from a companion star. Together with the transit light curves acquired by DBSP
and MuSCAT2, we are able to revise the system parameters and orbital ephemeris,
confirming that no transit timing variations exist. Our DBSP and MuSCAT2
combined transmission spectrum reveals an enhanced slope at wavelengths shorter
than 630 nm and suggests the presence of a cloud deck at longer wavelengths.
While the Bayesian spectral retrieval analyses favour a hazy atmosphere,
stellar spot contamination cannot be completely ruled out. Further evidence,
from transmission spectroscopy and detailed characterisation of the host star's
activity, is required to distinguish the physical origin of the enhanced slope.
