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the lines that run parallel with your eyes, the top of the roof, the top of the wall.
Then put up your pencil or ruler, holding it at one end and
Fig. 45.
parallel with your eye, and at arm's length. Close one eye. Raise or lower it until the roof or wall is almost but not quite covered. The pencil or ruler has a smooth unbroken edge, and every divergence from the straight line will be apparent.
