Sentence:  The draughtsman could not have held the sheet with the <WSD>apse</WSD> at the tip, for then, instead of shading away from the edge, most of his hatched lines would begin in the uncharted middle ground of a shadeable area, to strike against the contour; ... 
Meaning of the word marked with <WSD> tags:  A semicircular projection from a building, especially the rounded east end of a church that contains the altar.