that is, 1 and formula_18; this is equivalent to taking formula_19 or 3.26... as the value of π. but, wallis argued, we have in fact a series formula_20... and therefore the term interpolated between 1 and formula_21 ought to be chosen so as to obey the law of this series. this, by an elaborate method that is not described here in detail, leads to a value for the interpolated term which is equivalent to taking