A light source (15) is housed in a housing (6) of a case, underneath the dial (10) opposite a guide (17) directing the light onto a reflector (20), formed in a hollow in the internal or external face of a crystal (1) the thickness of which decreases regularly from the center to the periphery thereof for reorienting the light by total internal reflection into the crystal (1) until it emerges towards the dial (10) when the maximum refractive index is exceeded.