What rule was established by Mr. Justice Wright in Wilkinson v. Downton (1897)?

I believe that you are referring to the case of
Wilkinson v. Downton, in which Mr. Justice Wright presided.  I have
changed your question to reflect that.

The rule that was
established by this case has to do with what acts by a defendant make them liable for a
tort.  The rule says that any act which is calculated to cause harm and does cause harm
can be actionable, even if the harm is merely psychological.  This is why Downton was
liable for the mental distress caused to Mrs. Wilkinson by the lie that he told
her.

So, the rule in this case has to do with the
intentional infliction of emotional pain.  Wright held, for the first time, that this
was a tort similar to a physical assault.