§ 10-8.4 Rule against accumulations;  law  determining  validity  in exercise of a power of appointment When a direction for the accumulation of income  is  contained  in  an  instrument  exercising  a  power,  heretofore  or hereafter created, the  validity of such direction is determined by the law in effect  when  the  power  is  exercised,  and  not  by the law in effect when the power was  created.