It was not hard to discover his error, for many others before me had noticed it; but it is hard to elaborate, for none of his opponents have yet managed it, and perhaps I shall not either; I shall do my best all the same, so that even if I too am damned as a heretic, at least a more fortunate successor may find a usable preliminary work.

Above all I must emphasize most strongly that diverse refrangibility is not a fact [keine Tatsache, kein Faktum]. Newton himself narrates for us the course of his observations and his conclusions; the attentive critic is therefore able to follow close on his heels. Here I will only sketch in the outlines of the detailed presentation, which will be contained in the work itself [that was to follow]. […]