Phenylfurylmethylthiazolidine-2,4-dione and phenylthienylmethylthiazolidine-2,4-dione compounds of formula : are useful for promoting andor inducing andor stimulating the pigmentation of keratinous substances andor limiting their depigmentation andor their whitening and more particularly for preventing andor limiting canities of human keratinous fibers. According to the invention, a monitoring element driven with a constant current is provided. The device of the invention comprises: a signal source that generates a signal that varies in response to changes in angle or depth of the drill bit; and a holder to which the signal source is attached, the holder is mounted to the drill chuck of the drill and moves with its rotation. The light source is disposed in the insulative housing proximate the head portion.