()here seems to be no reason why the principles of estoppel by deed should not apply to assignment of a patent right….The grantor purports to convey the right to exclude others, in the one instance, from a defined tract of land, and in the other, from a described and limited field of the useful arts. The difference between the two cases is only the practical one of fixing exactly what is the subject matter conveyed.〔’’Westinghouse’’, 266 U.S. at 348-50.〕

It seems to be well settled that the assignor of a patent is estopped from saying his patent is void for want of novelty or utility, or because anticipated by prior inventions. But this estoppel, for manifest reasons, does not prevent him from denying infringement. To determine such an issue, it is admissible to show the state of the art involved, that the court may see what the thing was which was assigned, and thus determine the primary or secondary character of the patent assigned, and the extent to which the doctrine of equivalents may be invoked against an infringer. The court will not assume against an assignor, and in favor of his assignee, anything more than that the invention presented a sufficient degree of utility and novelty to justify the issuance of the patent assigned….〔''Westinghouse'', 266 U.S. at 351.〕