147. DISCHARGE TUBE PLACED IN OIL.—When the E. M. F. was increased, by having the discharge tube, as usual, in open air, sparks formed behind the electrode, and within the vacuum, and endangered the life of the discharge tube. He obviated this difficulty partly by having the electrode located well within the evacuated space, so that the wire leading to it was unusually long. By excessive E. M. F., also, streamers broke out at the end of the tube. To overcome all difficulties in connection with sparking and breaking of the tube, he followed the proposition of Prof. Trowbridge, and submerged the discharge tube in oil, § 11, at end, and § 13, which was continually renewed by flowing into and out of the vessel in which the discharge tube was contained, all as shown in the accompanying figure, p. 157, “Discharge Tube Immersed in Oil.” The discharge tube, _t_, may be recognized by its shape, and it is located horizontally in a cylindrical tube lying sidewise upon a table. To regulate the flow of the oil, the reservoir may be raised and lowered by a bracket, s. The X-rays enter the outside atmosphere by passing first through glass, then oil, and then through a diaphragm of “pergament” forming the right hand end of the oil vessel. When the results were compared with those obtained by Roentgen in his first experiments, the rays were found so powerful that it is not surprising that Tesla was able to obtain more definitely a closer knowledge of the properties of the rays. Roentgen obtained, with his tube and a screen of barium platino cyanide, a shadow picture of the bones of the hand at a distance of less than 7 ft., while Tesla obtained a similar picture with a screen of calcic tungstate, and with his tube immersed in oil at a distance of 45 ft. Tesla also made sciagraphs with but a few minutes’ exposure at a distance of 40 ft., by the help of Prof. Henry’s method, __i.e.__, with the assistance of a fluorescent powder. § 151. He referred also to Salvioni’s suggestion of a fluorescent emulsion. He attributed to Mr. E. R. Hewitt the conjecture that the sharpness of the sciagraphs might be increased by a thin aluminum sheet having parallel groves. Several experiments were made, therefore, with wire gauze, as well as with a screen formed of a mixture of fluorescent and iron-fluorescent powders. With the strong power of the rays as obtained by Tesla in combination with such adjuncts, the shadows were sharper, although the radiation, of course, was weakened by the obstruction. § 107_b_. [Illustration: DISCHARGE TUBE IMMERSED IN OIL, § 147, PAGE 156. ]