118 Prof. C. Barus on Elliptic and other 



lowest key of the instrument a small weight sufficient to 

 keep the damper off the string. Then set this string vibra- 

 ting by plucking it with the fingers. The sound thus 

 produced will last about 10 or 15 seconds, but the string 

 will continue to vibrate for another 30 seconds or longer, 

 and during the latter part o£ the time no sound will be 

 audible. 



The string vibrates in two planes : one parallel to the 

 surface of the sound-board, and the other at right-angles to 

 it. We have seen that the vibration of the string is trans- 

 mitted to the sound-board most effectively by the pressing 

 and pulling of the string on the bridge at right-angles to 

 the surface of the sound-board. The vibration of the string- 

 in this plane is rapidly absorbed by the sound-board and 

 therefore the audible sound rapidly ceases, but the vibration 

 parallel to the surface of the sound-board does not set the 

 latter in vibration to any appreciable extent, and therefore 

 the vibration in this plane continues a much longer time and 

 no sound is heard. By sliding a straight-edge along the face 

 of the adjoining strings and gradually bringing it over the 

 vibrating string, it will be found that the latter will hardly 

 touch the straight-edge, thus showing the vibration to be 

 in one plane. 



14 Citv Road, London, E.C. 

 March 23, 1911. 



VII. Elliptic and other Interference with Reflecting Gratings. 

 By C. Barus, Ph.D., LL.D., Professor of Physics at 

 Drown University, Providence, U.S.A.* 



1. JpiRST METHOD.— There are two or three typical 

 cases in the use of reflecting gratings for the pro- 

 duction of interferences in the spectrum, each of which 

 shows peculiarly interesting features. The first of these is 

 given in fig. 1, and corresponds closely to the method 

 described for transmission gratings in a preceding paper f. 

 If L is the source of light and M a glass plate grating, it 

 was shown that plane mirrors in the positions G m and G W5 

 each reflecting a spectrum from M, produce elliptical inter- 

 ference whenever the rays returned after passing M by 

 transmission and reflexion, respectively, are made to overlap 

 in the spectrum, under suitable conditions. 



* Communicated by the Author. Abridged from a Report to the 

 Carnegie Institution of Washington, D.C. Cf. Note in ' Science/ 

 January 20, 1911. 



t Phil. Mag. July 1910. 



