62 Applied BiopJiysics 



between X-rays and Ansible light. Deep-blue light with a wave 

 length of OAS[i represents the shortest wave length visible to 

 the eye, while red light with a wave length of from 0.63[i to 

 0.70u represents the longest visible wave length. Infrared 

 radiation describes wave lengths from 0.70[i to 20|.i or more, 

 and these merge imperceptibly into the short electric or radio 

 waves. The wave lengths used in radiant heat treatment are 

 from the visible red up to, say, 20[.i. 



In addition to the generalized heating produced when electro- 

 magnetic radiation is absorbed, specific efifects may be produced, 

 and these have been explained by the quantum theory. This 

 theory states that radiation is not to be considered as a con- 

 tinuous flow of energy but as a shower of minute energy parcels 

 or quanta, each representing an energy contribution of a definite 

 amount. Emission and absorption of radiation can only take 

 place in whole or multiple quanta, never in fractions of a quan- 

 tum. The energy contribution of each quantum in radiation 

 of a given wave length is inversely proportional to the wave 

 length, i.e., the energy parcels of X-rays are larger than those 

 of ultraviolet rays, and these in turn are larger than those of 

 visible light or infrared radiation. The production of certain 

 intramolecular changes, for example, those leading to the pro- 

 duction of vitamin D in the tissues, requires the action of quanta 

 of a certain minimum size peculiar to this particular change, 

 that is, this change can be produced only by light of a wave 

 length sufficiently short to give quanta of the necessary size. 

 The application of radiation of longer wave length will not pro- 

 duce the same efifect, even if large amounts of energy are 

 supplied, simply because this longer wave radiation contains 

 no quantum of the necessary size. The efiicacy of X-ray and 

 ultraviolet therapy depends on this fact. They are administered 

 in small doses — only 10 gram calories or less at a treatment — 

 and produce specific chemical changes in the tissues. They also, 

 of course, produce heating of the tissues, but this is so slight as 

 to be masked completely by the specific changes, even though 

 the latter are caused by only a small proportion of the total 

 incident energy. 



