"Defendant contends that the trial court erred in refusing to give a requested instruction dealing with the credibility to be given to the testimony of police officers, detectives or others employed to detect crime. The only witnesses to whom the instruction could apply were R. H. McDonald and Calvin H. Goddard, called by the state as expert witnesses for the purpose of proving that the bullet found in the body of the deceased was fired from the gun that defendant had in his possession at the time of the murder. We know of no cases that hold that expert witnesses, hired to give technical evidence to a jury, are within the scope of the rule for which defendant contends. The purpose of a rule requiring the trial court to instruct, upon request, relative [*57] to the credibility [***9] and weight to be given to the evidence of private detectives and informers is to protect the person charged with crime from the overzealous efforts of such persons in obtaining evidence, especially when they are paid only for the results of their labors. We know of no reason why an expert, testifying as to his technical knowledge of a subject involved in the litigation, should be placed within the scope of such a rule. The trial court rightfully refused to give the proffered instruction."