Leonardo Mirror Grinding Machine
Leonardo mirror grinding machine (or lens grinding machine) is a masterpiece of mechanical engineering among his inventions. It has several gears in mesh at all times and also has several variable settings built into it.
This mirror grinder was most likely designed by Leonardo while he was either investigating the different ways that light reflected off things or maybe he was planning on inventing a telescope or some other optical device.
Here is the video showing exactly how it works –
Devices like these are still used nowadays by amateur telescope/optical instrument makers around the world. The version they use is a tabletop sized mechanism, but is essential of the same design as Leonardo da Vinci mirror grinding kit, the only difference being the use of an electric motor instead of the hand crank