In the 1862 law the amount of income declared by a person in the employ or service of the United States was determined by

Because the government directly paid those in its employ or service (such as Members of Congress), it could order those issuing the pay to deduct and withhold the tax:

"Sec. 86. And be it further enacted, That on and after the first day of August, eighteen hundred and sixty-two, there shall be levied, collected, and paid on all salaries of officers, or payments to persons in the civil, military, naval, or other employment or service of the United States, including senators and representatives and delegates in Congress, when exceeding the rate of six hundred dollars per annum, a duty of three per centum on the excess above the said six hundred dollars; and it shall be the duty of all paymasters, and all disbursing officers, under the government of the United States, or in the employ thereof, when making any payments to officers and persons as aforesaid, or upon settling and adjusting the accounts of such officers and persons, to deduct and withhold the aforesaid duty of three per centum, and shall, at the same time, make a certificate stating the name of the officer or person from whom such deduction was made, and the amount thereof, which shall be transmitted to the office of the Commissioner of Internal Revenue, and entered as part of the internal duties;..." (Revenue Act of 1862)