Text in force at 23-63: from 23-0, effective 1 Feb 1965.
The instrument lights must--
(a) Make each instrument and control easily readable and discernible;
(b) Be installed so that their direct rays, and rays reflected from the windshield or other surface, are shielded from the pilot's eyes; and
(c) Have enough distance or insulating material between current carrying parts and the housing so that vibration in flight will not cause shorting. A cabin dome light is not an instrument light.
29 FR 17955 · Source