168 MBMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



certain that these images yield nothing to those of Feddersen in fineness of detail. Nyland in this 

 paper also first describes the method of obtaining the vibratory period of a fork by passing the 

 secondary spark of an iuductorium from a style, fastened to the fork, to the smoked paper covering 

 a revolving metallic cylindei". The primary circuit was closed and opened each second by a clock. 

 He, however, did not experiment to bring this method to give precise results. 



Helmholtz, 1869, Verhandlungen des naturhistorischen medizinischen Vereitis zu Heidelberg, ob- 

 served that the discharge of an inductorium, with a Leyden jar in the secondary circuit, into 

 the nerve of a frog caused 45 maxima antl minima of contractions, but that these vibratory phe- 

 nomena were not observed when the Leyden jar was absent. 



Rood, 1872, American Journal of Science, observed the multiple character of the discharge of an 

 iuductorium, with a Leyden jar in the secondary circuit, by means of a revolving mirror and a ro- 

 tating disk. The revolving disk was formed of two superposed disks, each with a radial slit. By 

 rotating the smaller disk an angular separation of the slits could be obtained, so that when the re- 

 flection of the discharge from white paper was viewed through these slits the images of the multiple 

 slits could be brought together, so that the end of one set of images given by one slit just touched 

 the set of images given by the other slit. Knowing the velocity of rotation of the disk and the 

 angle separating its two slits, the duration of the composite discharge was obtained. He thus ob- 

 served as many as 10 to 20 separate sparks in a discharge (with jar of 114 square inches of surface 

 in circuit) whose duration was about ^^ of a second. With platinum points as electrodes, and sep- 

 arated by 1, 2, 3, 4, and 5 millimeters, the number of sparks observed at these distances of the 

 electrodes were respectively, 4, 3, 2, 1, and 1. With ajar of only 11 square inches of surface, and 

 the electrodes formed of brass balls, the discharge was more complex. Observing in the revolving 

 mirror he saw the discharge formed of a bright spark followed by a violet discharge, and the latter 

 followed by four sparks. The total duration of this discharge was -^g of a second. The violet portion 

 lasted j^o of a second, and the four sparks lasted Yxftrg of a second. Other forms were sometimes 

 present, consisting of a faint violet streak teruiinated at each end by a spark, the whole duration 

 being ^ of a second. On increasing the striking distance between the balls to 2, 3, 4, .5, 0, 7, 9, and 

 10 millimeters, the total number of sparks forming the discharge for the striking distances was, 

 respectively, 5, 8, 4, 3, 3, 3, 2, and 1 spark. In the series of papers (lSG9-'72) containing the above 

 results, Eood, by novel, refined, and precise methods, first succeeded in obtaining the duration 

 of one of the separate sparks which go to make up the number forming the composite discharge. 

 This he did by examining through a microscope a series of flue rulings on smoked glass of bright 

 and dark bands of equal breadths when illuminated by the discharge. These lines "ere so 

 fine that the magnified inuige of one of these measured 3^ of an inch. When viewed in the 

 revolving mirror they appeared as they did when the mirror was stationary, till the mirror ap- 

 proached 180 revolutions in a second. Then the lines grew fainter and fainter, and, finally, when 

 the mirror reached 183 revolutions in a second the lines disappeared by the mirror making the 

 reflection of a black band of the ruling to be displaced to its own width during the duration of the 

 spark. Eood thus found that the duration of this portion of the discharge was .000000175 of a 

 second. With a jar of only 11 square inches of surface in circuit, and by using finer rulings and 

 a more rapidly revolving mirror he determined that the spark lasted only forty-eight billionths 

 of a second. "With this light" (lasting only forty billionths of a second) "distinct vision is possiltle. 

 Thus, for exami)le, the letters on a printed page are plainly to be seen. Also, if a i)olarisc<)pe be 

 used, the cross and rings around the axis of crystals can be observed, with all their peculiarities, 

 and errors in the azimuth of the analyzing prism noticed. * * * AH of which is not so 

 wonderful, if we accept the doctrines of the undulatory theory of light, for, according to it, in forty 

 billionths of a second nearly two and a half millions of the undulations of light reach and act on 

 the eye." 



Cazin, 1873, Journal de Physique, observed the multiple character of the discharge of induc- 

 torium with an ajjparatus similar to Rood's. 



Mayer, 1874, American Journal of Science, in this paper he refers to the previous work of 

 Henry, Feddersen, Rood, and Cazin. He used a large inductorium, having a "striking distance" 

 of 21 inches. With electrodes of platinum points, one millimeter apart, and no jar or condenser iu 

 circuit, he found the discharge of this inductorium to consist of 33 sparks, lasting ^ of a second. 



