The Scanning Microscope 97 



tion. On the way to the specimen the oscillating electron pencil 

 passes an apertured fluorescent screen, and finally a fourth lens, 

 which is placed immediately in front of the specimen. This lens 

 has little influence on the scanning beam, but spreads out the 

 secondary electrons released in the specimen over a wide area 

 of the fluorescent screen which has a suitable positive potential 

 against the specimen. 



The beam current and the secondary electron current are 

 extremely small, of the order of 10"^^-10"^'* amp. The amplifica- 

 tion of such minute currents with ordinary amplifiers is hope- 

 less, the signal would be drowned by the noise which is mainly 

 caused by the thermal fluctuations of voltage in the input 



Fig. 36. R.C.A. scanning microscope, experimental form 



resistor, parallel with the first grid. Let this resistance be R, 

 the fluctuation voltage v, Boltzmann's constant k, the tempera- 

 ture of the resistor T, and the frequency band transmitted by 

 the amplifier AF. Nyquist's well-known formula gives for the 

 mean square voltage fluctuation 



v^ = AkTR^F (28) 



The R.C.A. reduces this noise by two ingenious artifices. The 

 first is employing an electron multiplier as the first stage of the 



