Sixth Exception.-It is not defamation to express in good faith any opinion respecting the merits of any performance which its author has submitted to the judgment of the public, or respecting the character of the author so far as his character appears in such performance, and no further. Explanation.-A performance may be submitted to the judgment of the public expressly or by acts on the part of the author which imply such submission to the judgment of the public. Illustrations (a) A person who publishes a book, submits that book to the judgment of the public. (b) A person who makes a speech in public, submits that speech to the judgment of the public. (c) An actor or singer who appears on a public stage, submits his acting or singing to the judgment of the public. (d) A says of a book published by Z- "Z's book is foolish: Z must be a weak man. Z's book is indecent; Z must be a man of impure mind." A is within this exception, if he says this in good faith, inasmuch as the opinion which he expresses of Z respects Z's character only so far as it appears in Z's book, and no further. (e) But if A says "I am not surprised that Z's book is foolish and indecent, for he is a weak man and a libertine." A is not within this exception, inasmuch as the opinion which he expresses of Z's character is an opinion not founded on Z's book.