§ 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.