Letters and cards of Sir Peter Higgs to Lucinda Mackay regarding their friendship; social invitations and engagements; his family life and work; and the painting and exhibiting of Mackay's portrait of Higgs in 1981. Also includes a photograph of Peter Higgs taken in preparation for his sitting with Mackay; an invitation to Mackay to attend Higgs' 80th birthday party on 10 June 2009 at the Informatics Forum, Edinburgh; and a brief correspondence between Mackay and the Scottish National Portrait Gallery in 2010 in relation to the showing of the Higgs portrait as part of their re-opening exhibition in 2011.