The forms of the central and surrounding deities... should not be protruding like a clay statue or cast image, yet neither should they be flat like a painting. In contrast, they should be apparent, yet not truly existent, like a rainbow in the sky or the reflection of the moon in a lake. They should appear as though conjured up by a magician. Clear appearance involves fixing the mind one-pointedly on these forms with a sense of vividness, nakedness, lucidity, and clarity. 

-- Jigme Lingpa