Explanation:

Section 159 and 160
Section 159 - Refreshing memory A witness may, while under examination, refresh his memory by referring to any writing made by himself at the time of the transaction concerning which he is questioned, or so soon afterwards that the Court considers it likely that the transaction was at that time fresh in his memory. The witness may also refer to any such writing made by any other person, and read by the witness within the time aforesaid, if when he read it he knew it to be correct. When witness may use copy of document to refresh memory.—Whenever a witness may refresh his memory by reference to any document, he may, with the permission of the Court, refer to a copy of such document: Provided the Court be satisfied that there is sufficient reason for the nonproduction of the original. An expert may refresh his memory by reference to professional treatises.
Section 160 - Testimony to facts stated in document mentioned in section 159 A witness may also testify to facts mentioned in any such document as is mentioned in section 159, although he has no specific recollection of the facts themselves, if he is sure that the facts were correctly recorded in the document.