The exposition deviates with an explanation of how the black lines intersecting over a white ground (which give rise to various types of space already examined in the previous section), can be fundamentally but simply reversed to show white shapes floating above a black ground. This is a complete turning - inside - outside - operation, a Gestalt inversion where what was seen as background is seen as foreground and what was the foreground becomes the background. For this, the least quantity of black necessary makes the operation even more striking, inverting the foreground black lines to a background space with the foreground white floating over it. This might be done spontaneously - using a great quantity of black so that the remaining white might float naturally above it but to do the same using the minimum amount of darkness renders the operation more startling. The black originally appeared in front as intersecting lines but these lines can be undermined, being seen as fissures in a large underlying background.
Thus emphasizing two black forms in contact with a line at an intersection until they are seen as intervals between the white shapes immediatly establishes the black as underlying space, letting the white drift above. A further operation joining the isolated black lines to the black depths completes the task of lifting the white shapes. If we do not do so, we shall see some remaining black lines crossing the white shapes and preventing them from being detached. Eventually all of the black must exibit a continuity so the whites float above. This is a striking transformation - which requires a moment to be seen, but once seen it is unmistakable.
.XXII. BLACK ON WHITE, WHITE ON BLACK, S.W.
'New Ways of Gravure', fig 99, New York, Watson-Guptill Publications, 1981