South Haven, Mich., Aug. 7, 1907.
To the Editor:
—I take exception to the article with the above title in The Journal, Aug. 3, 1907, page 383. It is evident that Dr. Frank has left out of consideration the direct rays of light. On page 384, from Figure 2, the following distances in feet are obtained:AD 3.25 AC 2.50 DF 2.31 FC 2.43 FA 3.99 FB 3.14The direct illumination is approximately in the following proportion: (3.99)2 at C, (3.14)2 at B, and (2.43)2 at A. The direct illumination covers an angle of about 37.5 degrees and the indirect an angle of 225 degrees or six times the direct.Supposing the mirror to be nearly perfect, with little light lost by absorption, we get the following illuminating values:This gives an excess of light at C over A of more than 15 per cent. If Dr. Frank will place