Laudator Temporis Acti

B.L. Gildersleeve (1831-1924), "Brief Mention," American Journal of Philology 15.4 (1894) 520-523 (at 523):

In Brief Mention I often find myself adverting to the importance of typographical exactness. A man who makes remarks of that kind ought to abstain from article-writing and proof-reading. 'Un jour,' records that frivolous person, Jules Janin, Histoire de la littérature dramatique, III I72, 'on demandait à Geoffroy, pourquoi il ne faisait pas de comédies, lui qui les jugeait si bien? "Quand on donne le fouet aux autres, disait-il, il ne faut pas montrer son derrière."' And Brief Mention, as Nemesis will have it, is a veritable nidus of typographical errors.

It seems unnecessary to translate Janin's French, but in case someone can't understand it, here's a rough English equivalent:

One day someone asked Geoffroy why he didn't write comedies himself, since he was such a good judge of them. He answered, "When you give a whipping to others, you shouldn't expose your own backside."

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Karl Narveson writes:

I think "quand on donne" refers to a habit, not a single occasion, and therefore I question whether the "whipping" in your rough translation should be singular. "When you're in the business of administering whippings" might be more like it.