Abstract.D. K. Harrison has shown that if a ring with identity has a positive cone that is an infinite prime (a subsemiring that contains 1 and is maximal with respect to avoiding -1), and if the cone satisfies a certain archimedean condition for all elements of the ring, then there exists an order isomorphism of the ring into the real field. Our main result shows that if Harrison's archimedean condition is weakened so as to apply only to the elements of the cone and if a certain centrality relation is satisfied, then there exists an order isomorphism of the ring into a division algebra that is algebraic over a subfield of the real field. Also, Harrison's result and a related theorem of D. W. Dubois are extended to rings without identity; in so doing, it is shown that order isomorphic subrings of the real field are identical. It was hoped that [3, Proposition 7] could be extended so as to obtain representation of a certain class of rings as subrings of the division ring of real quaternions, for it is noted that many subrings of the quaternions satisfy our hypotheses. What we actually obtain is representation of such rings as subrings of division algebras that are algebraic over subfields of the reals. However, if the rings are commutative, the desired result follows; that is, they are then isomorphic to subrings of the complex field. In the general case, we can show that the semiring of nonnegative reals satisfies our hypotheses as a subsemiring of a ring A iff A is isomorphic to either the real field, the complex field, or the quaternions. This is quite analogous to Frobenius' theorem [8] which states that an algebraic real division algebra is isomorphic to one of these; indeed we use his theorem in our proof.