How does R. Eliezer arrive at these conclusions? Does he apply the signs to the beginning or to the end of each watch? If he applies his signs to the beginning of each watch, then it is unnecessary to have a sign for the first, as nightfall itself is a sufficient indication for it! If, however, he applies his signs to the end of each watch then the sign for the last is unnecessary, for the dawn is then sufficient! He applies his signs to the end of the first watch, the beginning of the last and the middle of the second. And if you please, you may say that he applies his signs to the end of each watch. In answer to your question as to the necessity of a sign for the last, I say: It is necessary for a man who sleeps in a dark place and does not know the time for reading the Sh'm'a; as soon as he hears the wife conversing with her husband, and the baby nursing from its mother's breast, he may begin to read the Sh'm'a.