It is research procedure that is scientific or not rather than its product, i.e. a theory. To be the scientific one, both experimental research and theoretical investigations must be systematic. There are three necessary conditions of the scientific character of research procedure. (1) The same set of laws should be applied in the theoretical analysis of different phenomens. (2) In different applications of the set of laws there should appear the same groups of additional conditions in recurring configurations . (3) While choosing additional conditions the whole knowledge about supposed elements of the system under study should be taken into account. The general principle is that if nothing is known that a theoretical property of a given element can change, it should be ascribed to the same element present in subsequent experiment. The author tries to show that all those conditions were satisfied in case of Roentgen's 1895 research.