Section 78. Mitigating circumstances in action for libel or slander

In an
  action  for  libel  or  slander  the  defendant  may  prove   mitigating
  circumstances,  including the sources of his information and the grounds
  for his belief, whether or not he has pleaded or attempted to prove  any
  defense.  A  defendant  in  default for want of an answer may prove such
  mitigating circumstances upon a reference or inquiry  to  ascertain  the
  amount  of  the  plaintiff's damages. Matter tending only to mitigate or
  reduce damages is a partial defense and may be set forth in the answer.