Could this optical instrument be the secret behind the accurate and realistic drawings of Ingres and other great masters?

Although the camera lucida was known about for hundreds of years, it was David Hockney who can be credited with bringing it to the attention of the world in the 21st century. 

His bestselling book 'Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters' sets out his theory and evidence that many artists of the past have used optical devices as an aid to achieving accuracy and realism.

The front cover shows him at work using the instrument.

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