[timewizard]

A candle is an exceptionally unstable light source. The flame continually moves location as it burns. Any air currents in the room will disturb it and cause flame shadows to be cast. Adding magnification will just make this all worse.

[ForOldHack]

Henry Cavindish used them to measure G. I used lasers and never got even an order of magnitude closer.

[timewizard]

His measurement apparatus was designed to eliminate air currents was it not? He also had a torsion balance and a vernier to make measurements of the /relative/ changes induced in the apparatus.

I don't see how you're going to use this system to effect repeatable markings on glass tubes.

[oddmiral]

These problems are easy to solve in practice: just put the candle into a lantern.

Try it: it's easy. Use a flashlight from your phone instead of a candle, then project light to a wall, then put your palm in the front of the light.