(1) When part of an act, declaration, conversation or writing is given in evidence by one party, the whole on the same subject may be inquired into by the other. When a letter is read, all other letters on the same subject between the same parties may be given. When a detached act, declaration, conversation or writing is given in evidence, any other act, declaration or writing which is necessary to make it fully understood, or to explain the same, may also be given in evidence.

(2) The judge may in his discretion either require the party thus introducing part of a total communication to introduce at that time such other parts as ought in fairness to be considered contemporaneously with it, or may permit another party to do so at that time.