Sir John Salmond stated in the first edition of his Law of Torts (1907) that a wrongful act is deemed to be done by a servant in the course of employment if it is “either (a) a wrongful act authorised by the master, or (b) a wrongful and unauthorised mode of doing some act authorised by the master”. This passage, to be found at page 443 of Salmond & Heuston on the Law of Torts, 21st ed. (1996), has been cited with approval in many judgments. The first alternative is unproblematic, save that it refers to a situation where the master is primarily rather than merely vicariously liable. But the second can be difficult to apply, particularly where the servant’s tort is intentional. How can it be right to describe conversion by a servant of a fur which he has been told to clean (Morris v. C.W. Martin & Sons Ltd. [1966] 1 Q.B. 716) or deceit of his master’s client (Lloyd v. Grace, Smith & Co. [1912] A.C. 716) as a mode of doing an authorised act, when it is effectively the opposite of what he has been authorised to do? The problem is even more acute when the tort consists of trespass to the person. In Trotman v. North Yorkshire County Council [1999] L.G.R. 584 the Court of Appeal held the defendant Council not liable for a sexual assault by a teacher on a handicapped teenager committed to his care on a foreign holiday, on the ground that it was an independent act outside the course of employment. Trotman has now been overruled by a unanimous House of Lords in Lister v. Hesley Hall Ltd. [2001] 2 W.L.R. 1311, where it was held that the proprietors of a school for children with emotional and behavioural difficulties were vicariously liable for the systematic sexual abuse of two teenage boys by the warden of a boarding house.