Using light to establish direction was indeed a technique used in the construction industry, and it was very widely used. Modern levels were essentially the same thing, only more advanced and electronic. They could even emit infrared light to project a cross marking the vertical and horizontal lines.
However, that carpenter clearly had no idea that such high-tech devices as levels existed. What he had come up with was actually different from a level.
After drawing a rough sketch on paper, the man explained it to everyone.
"Look, we can use a bamboo tube open at both ends and lower it from above the vertical shaft. A Calcium Carbide Lamp can be installed at the upper end as a light source. The lamp's light can shine down through the bamboo tube, and then a Glass Mirror can be installed at an angle beneath the tube to reflect the light coming from above straight ahead.
"It would be best to make a bend here below, turning the light shining down from above into a beam directed forward. That way, the workers underground can use the point of light shining ahead as their direction of advance and dig toward it. As long as the lamp doesn't move, the digging direction definitely won't deviate!" the man said with absolute certainty.
Everyone looked at the sketch he had drawn and immediately understood the principle. They could not help clicking their tongues in amazement, thinking that the young fellow was clever indeed to have come up with such an idea.
What none of them had expected, however, was that this thing was actually the prototype of a periscope. By reflecting light with two coaxial lenses set at 45-degree angles, it reflected the light from above down below. The periscopes used on early tanks and submarines