The structure of the human eye has a concave retina, rather than a flat sensor. This produces effects observed by Abraham Bosse, who in his 1665 illustration 'To prove that one can neither define nor paint as the eye sees', demonstrates how the circular projection of the visual cone conflicts with the flat plane of the picture surface, prompting continued debate over whether straight lines in the world are perceived as straight or curved in a form of barrel distortion, and whether they should be depicted as straight in the picture plane.