I'll Be Waving As You Drive Away: Part 1

S4 E21: Long hours studying by candlelight for the state teacher's exam results in tired, blurry eyes for Mary Ingalls, prompting a routine visit to the eye doctor; but when Charles is told that Mary's worsening sight is just the beginning of a much more serious problem, the concerned father can't bring himself to tell his beloved daughter that she will, most certainly, soon be completely blind.