. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. SOME CURIOSITIES OB1 VISION. 199 and sometimes, but only with very great difficulty, even a fourth time. He gave to this phenomenon the name of recurrent vision; it may perhaps be more appropriately denominated the Young effect. We have here a machine XDresented to the institution by Mr. Wiins- hurst, which is a giant in comparison with that used by Professor Young, and I hope by its means to be able to show the effect to every- one present
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. SOME CURIOSITIES OB1 VISION. 199 and sometimes, but only with very great difficulty, even a fourth time. He gave to this phenomenon the name of recurrent vision; it may perhaps be more appropriately denominated the Young effect. We have here a machine XDresented to the institution by Mr. Wiins- hurst, which is a giant in comparison with that used by Professor Young, and I hope by its means to be able to show the effect to every- one present who will give a. little attention. Look in the direction of some object which is exposed to the light of the discharge; the object will be seen for an instant at the moment when the spark passes and you hear the crack, and after a dark interval of about one-fifth of a second it will make another brief appearance. Some of you may per- haps see even a second recurrent image. Under certain conditions I myself have observed no less than six reappearances of an object which was illuminated by a single dis- ^^- *" charge. Twelve years ago I called attention to a very different method of exhibiting a recurrent image. The apparatus used for the purpose consists of a vacuum tube mounted in the usual way upon a horizontal axis capable of rotation. When the tube is illuminated by a rapid succession of discharges from an induction coil, and is made to rotate very slowly (at the rate of about one turn in two or three seconds) a very curious phe- nomenon may be noticed. At a distance of a few degrees behind the tube, and separated from it by a clear interval of darkness, comes a ghost. This ghost is in form an exact reproduction of the tube; it is very clearly defined, and though its apparent lumi- nosity is feeble, it can no doubt be easily seen by most of you. The varied colors of the original are, however, absent, the whole of the phantom tube being of a uniform bluish or violet tin
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