The following sentence from The Great Gatsby strikes me as a mixed metaphor.
The exhilarating ripple of her voice was a wild tonic in the rain.
The author is thus calling her (Daisy's) voice both an "exhilarating ripple" and "a wild tonic." To me, those ideas clash.
A colleague insists that this is a beautiful description of Daisy's voice and not, in fact, a mixed metaphor.
The definition of mixed metaphor, from Merriam-Webster online:
a metaphor that combines different images or ideas in a way that is foolish or illogical
And from Google dictionary:
a combination of two or more incompatible metaphors, which produces a ridiculous effect
Clearly, the sentence in The Great Gatsby combines different images or ideas, so is it simply a matter of personal opinion whether this combination is "foolish" or produces a "ridiculous effect"?