In order to devise means for testing the comparative reflecting power in a more decided manner, he laid pieces of different metals side by side upon a lead plate. Consequently the reflecting surface was formed of two parts corresponding to the two metals. § 80. The vertically perpendicular partition of lead served to prevent the mingling of the rays from the two metals. Ingenious precautions were taken; as for example, so arranging matters that upon equal areas of the two plates, equal amounts of X-rays impinged. § 80. He undertook to determine the position of iron in the series by thus comparing it with copper. It was impossible to be sure which metal reflected better. The same regarding tin and lead and also in reference to magnesium and zinc. Here, a difference was noticed, namely that the magnesium was a better reflector. He has made practical application of the power of the substances to reflect a certain per cent. of the rays by employing reflectors for the purpose of reducing the time required for exposure of the photographic plates. It admits, he stated, of the use of reflectors in combination with a whole set of discharge tubes, whereby rays which would be otherwise scattered in all directions are brought more nearly to a single direction of propagation. [Illustration: FROM SCIAGRAPH OF KNEE-JOINT. STRAIGHT, FRONT VIEW. By Prof. Goodspeed. _Photo. Times_, July, ’96. ] It might be argued, that in as much as zinc would reflect only about three per cent. of the incident rays, no practical gain would ensue in sciagraphy by the use of a reflector. He pointed out the falsity of such an argument. In the first place, the angle employed in these tests was 45°. With greater angles, the proportion of reflected rays would be greater assuming that the law of reflection is the same as that of light. By mathematical calculation and tests, he showed that there was no doubt whatever about the advantage of using reflectors. He obtained a sciagraph, on a single plate, of the ribs, arms and shoulder, clearly represented. He stated the details as follows. “A funnel shaped zinc reflector two feet high, with an opening of five inches at the bottom and 23 inches at the top, was used in the experiment. A tube similar in every respect to those previously described, was suspended in the funnel, so that only the static screen of the tube was above the former. The exact distance from the electrode to the sensitive plate was four and one-half feet.”