 all the before @-@ mentioned courts of the United States shall have power to issue writs of . . . habeas corpus . . . . And that either of the justices of the supreme court . . . shall have the power to grant writs of habeas corpus for the purpose of an inquiry into the cause of commitment . — Provided , That writs of habeas corpus shall in no case extend to prisoners in gaol , unless where they are in custody , under or by colour of the authority of the United States , or are committed for trial before some court of the same , or are necessary to be brought into court to testify . 
