The theory is that alot of the old masters at one time used this device to produce an outline for their paintings.
In the day where artists where art was a profession that lead to much wealth. This device was enough to get you banned in the artist's circles for being a cheat.
Was it true did artists use a device like this to cheat?
Type Old Master and optic anywhere on the internet and about a million books will pop up with the claim to the answer.
We look to the old masters style and use of color and light still today as a benchmark. And people spend half their lives trying to duplicate what the old ones started. Are we looking at a trick or was it truly their skills that made us want to know the secret of how they became great.