Commentary Par XXVIII 4-9

This return to the conditions of the experiment alluded to in the second canto (see C.Par.II.94-105) shows how captious some readers are in their insistence that Dante deliberately presents that experiment as being literally impossible.  Such a reader will once again object that, if the flame is behind the subject's back, it cannot be reflected in a mirror set directly in front of him.  And once again a less positivistic reader will realize that, if the flame is, for instance, only a few centimeters above the observer's head (as it is in the reproduction of a fifteenth-century illustration of the experiment [see Boyd.1995.1, p. 15)]), the result will be as Dante says.  In any case, this is a poem and not a physics lab.  And yet we should realize that Dante only says 'behind' (dopo, retro) the observer, without in any way suggesting that the flame might not be visible from a point directly in front of him.