Comments:

Alex Johnson emigrated to Canada in 1857 and was a professor of Mathematics and Natural Philosophy at McGill in Montreal. He later attended Trinity College in Dublin, and received a Doctor of Laws degree in 1861. He later returned to Canada, to eventually serve as the Dean of the Faculty of Arts at McGill. The letter was likely to congratulate Johnson for receiving his doctorate in 1861, noting the "LL.D." on the mailing.