HELMHOLTZ IN BERLIN 



attached to a suitable part of the apparatus that 

 was in motion. But he was not the inventor of 

 this method, as has sometimes been erroneously 

 stated. PoggendorfF, in Vol. VII. of his Annalen^ 

 issued in 1826, describes an experiment in which he 

 attached a mirror to a magnetised bar, and from 

 this mirror a beam of light was reflected into a 

 theodolite. This was the first example of making a 

 beam of light act as a weightless pointer, and thus to 

 amplify and indicate movement upon a scale. The 

 method has been of the greatest service to physical 

 science, and is seen to perfection in the arrangements 

 of Lord Kelvin's electrometer and galvanometer. 



Newton established that white light consists of 

 innumerable different homogeneous constituents which 

 are dispersed by refraction. This is proved by passing 

 white light through a prism, when a spectrum is pro- 

 duced. The index of refraction becomes greater and 

 greater as we pass through red to orange, orange to 

 yellow, yellow to green, green to blue, blue to indigo, 

 and indigo to violet, the violet rays being often described 

 as the most refrangible of visible rays. It is also 

 known that if certain media are placed in the path of 

 the beam of light, and the light is then passed through 

 a prism, certain parts of the spectrum show dark bands, 

 known as absorption bands. This is well seen when 

 a layer of blood sufficiently diluted is examined with 

 a spectroscope. Two absorption bands may then 

 be detected, one next the Frauenhofer line D, 

 227 



