In McClurg v. Kingsland, 42 U.S. 202, 1 How. 202 (1843), the U.S. Supreme Court held that there is no constitutional barrier to the legislative expansion of existing patents.
The patentee in this case was unprotected under the law in force when the patent issued because he had allowed his employer briefly to practice the invention before he obtained the patent. Only upon enactment, two years later, of an exemption for such allowances did the patent become valid, retroactive to the time it issued. McClurg upheld retroactive application of the new law.
The Court explained that the legal regime governing a particular patent "depends on the law as it stood at the emanation of the patent, together with such changes as have been since made; for though they may be retrospective in their operation, that is not a sound objection to their validity." 1 How. at 206.
- Neither is it a sound objection to the validity of a copyright term extension, enacted pursuant to the same constitutional grant of authority, that the enlarged term covers existing copyrights.