apostrophe - Correct usage of the possessive in the name "Christiaan Huygens"

I'm writing a lab for a simple diffraction experiment, and I am stuck on this grammatical point:

Is it more appropriate to say "Huygens' Principle" or "Huygens's Principle"*? A cursory glance at Google seems to indicate that the most common usage seems to be "Huygens' Principle"; but as the apostrophe is at the end of the word as in the plural possessive, my inner grammarian is screaming that it should be "Huygens's". Which is correct?

*I know it can also be rendered as the Huygens-Fresnel Principle, obviating the need for my query.

Answer

There is a degree of ambivalence between Huygens' and Huygens's. The Chicago Manual of Style says that either may be used, whereas Professor Strunk (The Elements of Style) insists it is Huygens's.

I personally prefer Huygens's, as it seems to more explicitly indicate the possessive. Your mileage may differ.

See Wikipedia on English possessive.