_Y._ Yellow impression; negative made through a blue-violet filter. _R._ Crimson impression; negative made through a green filter. _RY._ Crimson on yellow. _B._ Blue impression; negative made through a red filter. _YRB._ Yellow, crimson, and blue combined; the final product. (Courtesy of Phototype Engraving Co., Philadelphia.)] WILLIS E. TOWER, M. SCI. (Univ. of Illinois) HEAD OF THE DEPARTMENT OF PHYSICS, ENGLEWOOD HIGH SCHOOL, CHICAGO…
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