  The Lyman decrement associated with the cumulative effect of HI in QSO
absorption systems along the line of sight provides a distinctive feature for
identifying galaxies at z>2.5. The Hubble Deep Field (HDF) observations offer
the opportunity to exploit the ubiquitous effect of intergalactic absorption
and obtain useful statistical constraints on the redshift distribution of
galaxies considerably fainter than current spectroscopic limits. We model the
HI cosmic opacity as a function of redshift, including scattering in resonant
lines of the Lyman series and Lyman-continuum absorption, and use stellar
population synthesis models with a wide variety of ages, metallicities, dust
contents, and redshifts, to derive color selection criteria that provide a
robust separation between high redshift and low redshift galaxies. From the HDF
images we construct a sample of star-forming galaxies at 2<z<4.5. While none of
the more than 60 objects in the HDF having known Keck/LRIS spectroscopic
redshifts in the range 0<z<1.4 is found to contaminate our high-redshift
sample, our color criteria are able to efficiently select the 2.6<z<3.2
galaxies identified by Steidel et al. (1996b). The ultraviolet (and blue)
dropout technique opens up the possibility of investigating cosmic star and
element formation in the early universe.
