Miniature Wood turning: the wood lathe chisel light

Why did I install a small light bulb on the shaft of a very small wood chisel? I use this on the wood lathe when I turn the inside of very small wine glasses. I turn out all the lights in the shop and the light coming through the wood acts as a dynamic thickness gauge as I remove wood. The intensity of the light through the wood (very thin wood) is inversely proportional to the thickness of the wine glass.