two wrongs don't make a right

"Foucault's pendulum," said Lewisham. "They use a rubber ball with a pin-hole hidden in the hand, and blow the pendulum round the way it ought to go." "But that's different," said Smithers. "Wait a moment," said Lagune, and produced a piece of folded printed paper from his pocket. "Here is a review from _Nature_ of the work of no less a person than Professor Greenhill. And see--a convenient pin is introduced in the apparatus for the demonstration of virtual velocities! Read it--if you doubt me. I suppose you doubt me." Smithers abruptly abandoned his position of denial "in toto." "This isn't my point, Mr. Lagune; this isn't my point," he said. "These things that are done in the lecture theatre are not to prove facts, but to give ideas." "So was my demonstration," said Lagune. "We didn't understand it in that light." "Nor does the ordinary person who goes to Science lectures understand it in that light. He is comforted by the thought that he is seeing things with his own eyes." "Well, I don't care," said Smithers; "two wrongs don't make a right. To rig demonstrations is wrong." "There I agree with you. I have spoken plainly with this man Chaffery. He's not a full-blown professor, you know, a highly salaried ornament of the rock of truth like your demonstration-rigging professors here, and so I can speak plainly to him without offence. He takes quite the view they would take. But I am more rigorous. I insist that there shall be no more of this...." "Next time--" said Smithers with irony. "There will be no next time. I have done with elementary exhibitions. You must take the word of the trained observer--just as you do in the matter of chemical analysis." "Do you mean you are going on with that chap when he's been caught cheating under your very nose?" "Certainly. Why not?"