Copy letter from Robert William Frederick Harrison to R Whiddington, St John's College, Cambridge 'We regret the misprint in your paper. It is however correct by copy. We have shown it to Sir Joseph Larmor, whose comment is "of no importance. I gather it merely means that B is negative". There seems to us no need to send you a second proof. You said "if you think it necessary".' 7 May 1912