40 ILCS 5/6-131

The amount of annuity for the wife of a present employee who attains age 57 or more on or before the effective date shall be fixed on the effective date as of the age of the wife at the time the employee attained age 57. The widow shall receive annuity, from the date of the employee's death of such amount as can be provided on a reversionary annuity basis from the employee's credit for such annuity on the effective date.