A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . into the Fig. 285. wall of the tube. When the secondary terminals of aninduction-coil in operation are connected with the electrodesof such a tube, transient currents of electricity pass throughthe gaseous space, producing curious and often very beauti-ful effects. Vacuum-tubes in which the glass itself has very compli-cated forms, and which are used largely to produce beautifulcolor effects, are called Geissler tubes, after a celebratedmaker of such apparatus. Tubes usin
A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . into the Fig. 285. wall of the tube. When the secondary terminals of aninduction-coil in operation are connected with the electrodesof such a tube, transient currents of electricity pass throughthe gaseous space, producing curious and often very beauti-ful effects. Vacuum-tubes in which the glass itself has very compli-cated forms, and which are used largely to produce beautifulcolor effects, are called Geissler tubes, after a celebratedmaker of such apparatus. Tubes using still higher vacua,provided with electrodes terminating in disks or portions ofspheres, and often with much other internal furniture ofmetal, mica, etc., are commonly called Crookes tubes,after an English investigator, whose experiments madesuch tubes widely known. • 412. Cathode Rays.—It was noticed long ago that thecathode of a vacuum-tube, the electrode by which the cur-rent leaves the tube, shows phenomena of peculiar highly rarefied tubes, like those of Crookes, some kind ELECTROMAGNE TISM. 527. of influence appears to proceed in straight lines from the cathode; for certain effects are observed at spots within the unobstructed reach of such lines and not at all or very little elsewhere. Thus Fig. 286 shows a shadow cast by a screen, S, placed in the course of rays from the cathode, C, within a Crookes tube. Crookes believed, and the opinion has now becomegeneral, that there is an actual projection of some materialfrom the cathode to the points affected. It is believedthat the particles of matter thus projected are smallerthan ordinary atoms (see foot-note § 372). About the year 1892 Hertz (§ 414) and Lenard foundthat cathode rays could penetrate, or at least produce aneffect through, opaque matter, such as thin films of metal. 413. Roentgen Rays.—In the latter part of 1895Professor Roentgen of Wurzburg discovered that from avacuum-tube in which the ca
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