¹* * * The Directors of the said Association shall have authority to call on the proprietors to pay such instalment or instalments as shall, together with the instalments already paid, amount to a sum not exceeding 50 per cent. on each share; and ¹* * * no further call shall be made, except in consequence of a vote of a general meeting of the proprietors, authorising such further call; provided always that no proprietor shall be called upon to pay more in proportion to his share in the Capital Stock than any other proprietor.