law: Indian Evidence Act, 1872 
  45. Opinions of experts 
  When the Court has to
  
  form and opinion upon a point of foreign law or of science or art, or as to
  
  identity of handwriting 30[ or finger impressions], the opinions
  
  upon that point of persons specially skilled in such foreign law, science or
  
  art, 31[ or in questions as to identity of handwriting ] 30[
  
  or finger impressions ] are relevant facts.
  Such persons are
  
  called experts.
  Illustrations
  (a) The question is,
  
  whether the death of A was caused by poison.
  The opinions of
  
  experts as to the symptoms produced by the poison by which A is supposed to
  
  have died, are relevant.
  (b) The question is,
  
  whether A, at the time of doing a certain act, was, by reason of unsoundness of
  
  mind, incapable of knowing the nature of the Act, or that he was doing what was
  
  either wrong or contrary to law.
  The opinions of
  
  experts upon the question whether the symptoms exhibited by A commonly show
  
  unsoundness of mind, and whether such unsoundness of mind usually renders
  
  persons incapable of knowing the nature of the acts which they do, or of
  
  knowing that what they do is either wrong or contrary to law, are relevant.
  (c) The question is,
  
  whether a certain document was written by A. Another document is produced which
  
  is proved or admitted to have been written by A.
  The opinions of
  
  experts on the question whether the two documents were written by the same
  
  person or by different persons are relevant.