54 Applied Biophysics 



is the demonstration in senile gangrene of increased blood flow 

 in the affected toe for some hours after the application of a 

 Parvex glass boot. 



Hertzman and Dillon -^ have applied photoelectric plethys- 

 mography to the vascular reactions, such as Raynaud's disease, 

 or in evaluating the completeness of sympathetic denervation 

 of the skin, etc. 



Radio-frequency Oscillators 



The main uses of these devices in medicine are diathermy 

 and short wave therapy. These are so well known that it is 

 not necessary to deal with them here. 



Radio-frequency probe. Farmer and Osborn ^^ describe an 

 apparatus for indicating the approximate position of metallic 

 substances. Theoretically, the instrument should be of value 

 in conjunction with X-ray examination. The principle is as 

 follows : A radio-frequency oscillator works on a frequency of the 

 order of 10*^ or 10^ cycles per second and the whole of the turning 

 inductance of this oscillator is in the form of a search coil capable 

 of being moved about near the patient. If the search coil ap- 

 proaches a metallic substance — such as a splinter in the operation 

 area — the inductance of the coil will change, and hence, the 

 frequency of the oscillator. The change of frequency can be 

 made audible by heterodyning these oscillations with those of a 

 second oscillator working on a slightly different frequency. 

 A beat note can be detected which can be heard through a 

 loudspeaker or headphones. 



The Electron Microscope 



The resolving power of a microscope is limited by the wave 

 length of light used. Moving electrons act as if they were asso- 

 ciated with a wave length. By using electron waves, 10~® of 

 the wave length of visible light, much greater resolution can be 

 got than with the optical microscope. 



The electron microscope is classified as follows : the magnetic 



