Dali copied some of his material from a postcard

Salvador Dalí (1904-1989) painted Forgotten Horizon (1936, Tate T01078) in 1936. One of a series of small oil paintings on wood panel, it depicts the beach at Rosas on the east coast of Spain with a group of alluringly posed dancers, meant to stimulate the imagination and subconscious. Recently, the work was in the Paintings Conservation studio at Tate for a technical examination...

The source for the dancers is reported to be a postcard, now lost. From the information gained by examining the painting with infra red light, conservators were able to determine that Dalí transferred the image on the card to the panel, outlining the figures, the internal contours and facial features.

Tate's digital infrared camera provides a series of small images which are pieced together into a mosaic...  Infrared light penetrates the upper layers of paint, and is either absorbed by the black media used for the underdrawing - such as pencil, ink or dilute black paint - or reflected by the white priming layer. This contrasting absorption or reflection is translated into a visible black and white image which reveals Dalí's preparatory outlines.

Further information is available at the Tate's Painting Conservation site.