636 



Tlie length of tlie siring, being about 10 millini. ('or waves of 

 (for instance) 10 kiloni., is only 1 niillim. for waves of 1 kilom. 

 We liave also experimented willi siiorter sti-ings showing u still 

 higher frequency of I heir proper vibiations. Heretofore as far as we 

 know it has not been [)0ssible to induce these frequencies in any 

 mechanisui. 



The string, for which we take a tine quartz fibre, is rendered able 

 to conduct by cathode bombardment, and stretched between two 

 microscopes; one of these serves to concentrate the light, the other 

 to project the image, whilst both microscopes, in order to obtain a 

 sharp definition of the string, must be veij near to one another. 

 The objectives, having a numerical apeitnre of 0,95, are no more 

 than 0,2 millim. away from the string. Since the front lens of such 

 an objective has a diameter larger than the leiigtii of the string, a 

 special device is necessary to fix the string; this is done in such a 

 manner that the rays of light are not intercepted, and the full angle 

 of aperture of the objectives is made use of efficiently. 



t"ig. 1. 



Diagram of the string s between both of tlie microscopes ilfj and Jfo. 



Bi and B.,. fine metal strips to which the string is soldered. The 

 direction of the rays of light is indicated by the dotted lines and 

 arrows. 



The difficulty was overcome by soldering both ends of the string 

 to fine metal strips |ilaced in the optical plane perpendicular to the 

 string, and rigidly attached to the apparatus in order to tighten and 

 slacken the string. 



It is important to have the string vibrating as freely as possible. 

 Therefore it has not only to be fine but also strongly stretched like 

 a string of a piano or a violin, lis minute mass per unity of length 

 causes it to suffer a strong damping effect from the air, and this 

 must be avoided. Therefore the space around it is evacuated, and 

 in order to make the xacuum efficient it has to be made high. We 



