2. No person under enlistment in the Army or Navy of the United States shall be examined under these rules except for some place in the Department under which he is enlisted requiring special qualifications, and with the consent in writing of the head of such Department. 3. The Commission may by regulations, subject to change at any time by the President, declare the kind and measure of ill health, physical incapacity, misrepresentation, and bad faith which may properly exclude any person from the right of examination, grading, or certification under these rules. It may also provide for medical certificates of physical capacity in the following cases, and for the appropriate certification of persons so defective in sight, speech, hearing, or otherwise as to be apparently disqualified for some of the duties of the part of the service which they seek to enter. RULE XVI. 1. Whenever any officer having the power of appointment or employment shall so request, there shall be certified to him by the Commission or the proper examining board four names for the vacancy specified, to be taken from those graded highest on the proper register of those in his branch of the service and remaining eligible, regard being had to any right of preference and to the apportionment of appointments to States and Territories; and from the said four a selection shall be made for the vacancy. But if a person is on both a general and special register he need be certified from the former only, at the discretion of the Commission, until he has remained two months upon the latter. 2. These certifications for the service at Washington shall be made in such order as to apportion, as nearly as may be practicable, the original appointments thereto among the States and Territories and the District of Columbia upon the basis of population as ascertained at the last preceding census. 3. In case the request for any such certification or any law or regulation shall call for those of either sex, persons of that sex shall be certified; otherwise sex shall be disregarded in such certification. 4. No person upon any register shall be certified more than four times to the same officer in the customs or postal service or more than twice to any Department at Washington, unless upon request of the appointing officer; nor shall anyone remain eligible more than one year upon any register; but these restrictions shall not extend to examinations under clause 5 of Rule VII. No person while remaining eligible on any register shall be admitted to a new examination, and no person having failed upon any examination shall within six months thereafter be admitted to another examination without the consent of the Commission.