With the thumb and index finger hold the two cups ( 27 ) le- with their retaining rings (28). The body of the delay device will position itself so that the slide- valve is in the middle of its stroke. 27 26 IV. REPLACING A QUARTZ - IODINE HEADLAMP BULB. PRECAUTIONS : This operation must only be carried out with the headlamps switched off. After switching off, it to let the head- lamps cool for 5 minutes before proceeding to handle them.