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In 1850, Foucault was the first to compare experimentally the velocity oflight in air to its velocity in water. And as the wave and projectile theoriescompeting at the time predicted inverse results to such a measurement, Foucault''declared the emission system incompatible with the reality of the facts''.Yet, in his Physical Theory, Duhem uses this very example to illustrate hisepistemological demonstration of the impossibility of the 'experimentum crucis'in physics. Then, the ambition of the present article is to augment Duhem'sdemonstration with historical evidence relating precisely to the question ofthe speed of light in water. We will review the four major opinions on thenature of light developed between 1637 and 1801; all leading to theoriesconcluding that the ratio of sines is constant and equal to the ratio of thevelocities in the two media, but all being defended by at least two authorsconcluding to inverse ratios of those velocities -- and thus to oppositepredictions on the result of an experiment such as Foucault's. The convergenceof historical evidence will then confirm that the nature of light, consideredin isolation, is in no way constrained by the measurement of its speed inwater.

Historical synthesis supporting Duhem's demonstration of the  impossibility of the `experimentum crucis' of comparison of the speed of  light in air and water