At S (point of sight) raise vertical SF·, making it equal to fL. F· becomes the foot of the luminary, whilst the luminary itself still remains at L.
Fig. 273.
We have but to turn this page half round and look at it from the right, and we shall see that SF· becomes as it were the horizontal line. The luminary L is at the right side of point S instead of the left, and the foot thereof is, as before, the trace of the luminary, as it is just underneath it. We shall also see that by
proceeding as in previous figures we obtain the same results on the wall as we did on the horizontal plane. Fig. B being on the horizontal plane is treated as already shown. The steps have their shadows partly on the wall and partly on the horizontal plane, so that the shadows on the wall are outlined from F· and those on the ground from f. Note shadow of roof A, and how the line drawn from F· through A is met by the line drawn from the luminary L, at the point P, and how the lower line of the shadow is directed to point of sight S.
Fig. 274 is a larger drawing of the steps, &c., in further illustration of the above.
Fig. 274.