What is the test for discovering discovery in English practice?

Under the English practice discovery is made by means of answering interrogatories. A party to the action who is interrogated is not excused from answering questions by saying he knew nothing about the matter and that his servant or agent does. He is bound to inform himself. It is laid down that the knowledge of the servant or agent is, under such circumstances, the knowledge of the employer, and the employer is bound to make reasonable efforts to obtain the information required. See Bolckrow v. Fisher, 10 Q.B.D. 161; 52 L.J.Q. B. 12.