Evolution of galaxies in clusters. I. ISIT photometry of Cl 0024 + 1654 and 3C 295

First results are presented of a program of two-color photometry of very distant (z greater than or equal to 0.4) clusters of galaxies, using the KPNO ISIT vidicon. It is demonstrated that the ISIT system is capable of accurate photometry of faint (V approx. 23.5) galaxies in exposure times of about 1 hour on a 2.1-m telescope. Photometry is presented for two clusters, Cl 0024 + 1654 (z = 0.39) and the cluster around 3C 295 (z = 0.46). Both clusters are rich, centrally condensed systems similar to the Coma cluster. However, in contrast to nearby clusters of this type, which contain only elliptical and S0 galaxies, between one-third and one-half of the galaxies in these two distant clusters have the colors of spiral galaxies. The possible implications of this result for the evolutionary history of S0 galaxies are discussed. 17 figures, 3 tables