19mm, H.M. & C.I. Button. Updated identification and with appreciation to Harry Eichman for supplying this Information: This button can be attributed to the Hamilton Military & Collegiate Institute, White Plains, New York formed in the late 1850's. The Institute was named for Alexander Hamilton who jointed the New York militia during the Revolutionary War, receiving a commission in the Continental Army, and raised an artillery company of his own. He proved his bravery at the Battles of Long Island and White Plains, where this school was located and it was later named in his honor. This is a rarely encountered uniform button  (Formerly the listing read:  The affiliation of this rarely encountered uniform button is unknown, but it's likely an academy or school in New York state and is inferred to read  "_____  Military & Collegiate Institute".   If you know the indentification of this unusual military school button, please reach out and share its attribution) Thank you, Harry, for sharing and advancing the knowledge base of our hobby.