(_a_) order the production of any document, exhibit, or other thing connected with the proceedings, the production of which appears to them necessary for the determination of the case, and (_b_) if they think fit order any witnesses who would have been compellable witnesses at the trial to attend and be examined before the court, whether they were or were not called at the trial, or order the examination of any such witnesses to be conducted in manner provided by rules of court before any judge of the court or before any officer of the court or justice of the peace or other person appointed by the court for the purpose, and allow the admission of any depositions so taken as evidence before the court, and (_c_) if they think fit receive the evidence, if tendered, of any witness (including the appellant) who is a competent but not compellable witness, and, if the appellant makes an application for the purpose, of the husband or wife of the appellant, in cases where the evidence of the husband or wife could not have been given at the trial except on such an application, and (_d_) where any question arising on the appeal involves prolonged examination of documents or accounts, or any scientific or local investigation, which cannot in the opinion of the court conveniently be conducted before the court, order the reference of the question in manner provided by rules of court for inquiry and report to a special commissioner appointed by the court, and act upon the report of any such commissioner so far as they think fit to adopt it, and (_e_) appoint any person with special expert knowledge to act as assessor to the court in any case where it appears to the court that such special knowledge is required for the proper determination of the case; and exercise in relation to the proceedings of the court any other powers which may for the time being be exercised by the Court of Appeal on appeals in civil matters, and issue any warrants necessary for enforcing the orders or sentences of the court: Provided that in no case shall any sentence be increased by reason or in consideration of any evidence that was not given at the trial. =10.= The Court of Criminal Appeal may at any time assign to an appellant a solicitor and counsel or counsel only in any appeal or proceedings preliminary or incidental to an appeal in which, in the opinion of the court, it appears desirable in the interests of justice that the appellant should have legal aid, and that he has not sufficient means to enable him to obtain that aid.