Evidence on failure of attesting witnesses

15. If both the subscribing witnesses are dead, or if from other circumstances such an affidavit cannot be obtained from either of them, resort for such an affidavit shall be had to other persons (if any) present at the execution of the will ; but if no such affidavit can be obtained, proof shall be required of that fact, and of the handwriting of the deceased and of the subscribing witnesses, and also of any circumstances raising a presumption in favour of the due execution of the will.