A student who enters a university in the second half of 20th century is in a new situation. He is not like the young man of the early 19th century who came to sit at the feet of his masters and left as a master. That situation no longer exists, because now it is not only the student who changes. He may grow fast, but science grows still faster. Today a student cannot master knowledge, he can only be its servant. He knows that he can become familiar (熟悉) with only a small comer of knowledge and that his learning will always be imperfect and imcomplete. But he can still hope to add something to the sum(总量) of knowledge, and so make the situation slightly more difficult for those who come after him. The phrase 'in the second half of the 20th century' means _________.