130. PREVENTING PUNCTURE AT THE PHOSPHORESCENT SPOT.—In experiments where he employed a flat cathode, a very thin pencil of rays of increased power came from the exact centre, and in two or three seconds made the glass red hot at the centre of the phosphorescent spot. Immediately, the atmospheric pressure perforated the bulb. This occurred several times. He stated that “the best remedy is to permit the central ray to strike the glass at a low angle; this greatly increases the area and prevents the trouble.” EDISON. Mr. Ludwig Gutmann furnished a translation of a note by Prof. Walter König, found in _Eleck. Zeit._ of May 14, ’96, relating to this same subject matter. Recognizing that the sharpness of the outlines is the most important requirement in connection with sciagraphy, and that if the rays start from a large surface the impressed shadows will be uncertain in configuration, and noticing, as Edison and Tesla did, § 130, the frequent destruction of the tube at the place where the rays were concentrated to a focus, he placed over the inner surface of the glass, aluminum foil for distributing the heat over a larger area, at the same time causing radiation of X-rays from a single point. The focus tube outweighs this in importance. § 91. 131. ELECTRICAL DIMENSIONS OF APPARATUS. The best kind of instruction for the student in reference to equipping a plant is to follow the construction employed by those who have been successful. § § 106, 109, 114, 137. Edison used the usual incandescent-lamp current, voltage at 110 to 120 volts, current being continuous, but not connected directly to the induction coil, there being a bank of eight to twenty 16 candle power incandescent lamps arranged in parallel. The interrupter for the primary consisted of a rotating wheel in appearance like a commutator of a dynamo, and was rotated rapidly by a small electric motor, making about 400 interruptions per second, and so constructed that the circuit was closed twice as long as it was open. A sudden interruption was caused by an air blast playing at the point of make and break, the use of which made that of a condenser needless. § 3. The discharge tube terminals were connected respectively and directly to those of the secondary. Prof. Pupin, Columbia Univ. N.Y. (_Lect. N.Y., Acad. Sci._, April 6, ’96, and _Science_, N.Y., April 10, ’96) gave valuable and practical instruction concerning the apparatus, which the author witnessed. “A powerful coil was found indispensable for strong effects and satisfactory work. The vibrating interrupter is too slow and otherwise unsatisfactory, and it was replaced by a rotary interrupter, consisting of a brass pulley, 6 inches in diameter and 1-1/4 inches in thickness. A slab of slate 3/4 inch thick was inserted and the circumference was kept carefully polished. This pulley was mounted on the shaft of a Crocker-Wheeler 1/8 H. P. motor giving 30 revolutions, and, therefore, 60 breaks per second. Two adjustable Marshall condensers