EFFECTS OF RADIANT ENERGY ON THE EYE. 703 



Theory of Action of Radiant Energy on the Tissues. 



A useful conception of the effects of radiant energy upon the tissues 

 of the body is that the heat effect is due to increased molecular motion 

 while the abiotic effect is due to direct atomic disintegration of the 

 molecules with immediately resulting chemical changes. The first 

 effect of the increased molecular motion is to produce a physical 

 change analogous, for example, to the melting of ice. When the 

 motion reaches a certain critical rate the molecules are broken up and 

 various chemical changes result. Both heat effects and abiotic 

 effects theoretically may be produced by rays. of any wave length, 

 but practically in the case of short waves the heat effect is generall\' 

 negligible, while in the case of long waves the abiotic effect is negli- 

 gible. Our experiments show that for human cells the abiotic effect 

 becomes negligible within a ^'ery short range of wave lengths, that is 

 between 305 and 310 /i/x. For bacteria it becomes negligible still 

 sooner, at less than 295 /x/i. Under ordinary conditions heat effects 

 are also negligible here, and in fact all through the visible spectrum, 

 although with extreme intensities such as afforded by concentrated 

 sunlight they may be produced, as in eclipse Ijlindness, for instance. 

 It is in reality due to the fact that abiotic effects and heat effects 

 are negligible in the region of the spectrum indicated, that sunlight 

 under usual conditions is not destructive to human life. This fact, 

 considered from the standpoint of evolution, suggests a relation of 

 light to the origin and structure of living matter, but a discussion of 

 this aspect of the subject would lead too far. 



Since according to this conception the abiotic action of light is 

 directly upon the structure of the molecules, slight chemical changes 

 are produced after almost infinitesimal exposures. Theoretically, of 

 course, there is a limit of exposure below which no disintegrating effect 

 is produced upon the molecules, so that a series of such short expo- 

 sures would produce no summative effect. Practically, however, this 

 would be impossible to demonstrate in the case of li\ing cells. On the 

 other hand, in the case of living cells summation of the effects of a 

 series of exposures, if the intervals were too long, would not accurately 

 occur, since the repair of the injury would take place to a greater or 

 less extent. Thvis we have found in the case of the corneal cells that 

 summation of effects becomes much less exact when the intervals 

 of exposure are over twenty-four hours. 



