From Law wiki, the wiki for law research

The Testator left instructions that his baronetcy should only be inherited by an heir who married a woman of the Jewish faith. According to his will, any doubts about the meaning of 'Jewish faith' were to be resolved by the Chief Rabbi. The question was whether this obligation was sufficiently certain that it could be enforced. The Court of Appeal held unanimously that it did.Lord denningaccepted that delegating the decision to the Chief Rabbi rendered the terms of the inheritance certain; Lord Russell considered the term 'Jewish faith' sufficiently certain that no reference need be made to the Chief Rabbi; Everleigh LJ held that the reference to the Chief Rabbi only need be construed as evidence of the settlor's view of what constituted Jewishness.

In consequence, although this case is often cited as authority for the proposition that certainty of objects can be ensured by delegating a decision to a particular person, it is not really clear that this was the Ratio Dicidendi of the case.