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The visual trick may have been created by the artist’s use of a common optical viewing aid called a camera lucida.

Some think he employed a camera lucida’s lens to render sitters as accurately as possible, which makes two details surprising.

Precisely ground, the mirrors were able to start fires and project images onto flat surfaces, camera lucida fashion.

Nash, who has donated photos and pre-photographic "camera lucida" pictures to the Getty and 20th century photos to the Los Angeles County Museum of Art, was not available for comment.

The cover glass is then removed and the wing mounted either on the same slide in balsam or floated to another slide, or at once accurately sketched with the camera lucida.