A written report or finding of facts prepared by an expert who is not a party to the cause, nor an employee of a party, except for the purpose of making such report or finding, nor financially interested in the result of the controversy, and containing the conclusions resulting wholly or partly from written information furnished by the co-operation of several persons acting for a common purpose, shall, in so far as the same is relevant, be admissible when testified to by the person, or one of the persons, making such report or finding without calling as witnesses the persons furnishing the information, and without producing the books or other writings on which the report or finding is based, if, in the opinion of the court, no substantial injustice will be done the opposite party.