EFFECTS OF RADIANT ENERGY ON THE EYE. G31 



action of the ultra violet. It, therefore, becomes of fundamental 

 importance to examine the effects of radiant energy with special refer- 

 ence to their relation to the wave length of the radiation. 



Nature and Distribution of Radiant Energy. 



All radiant energy is at present believed to consist of transverse 

 vibrations in the hypothetical ether, all propagated at the same rate 

 and differing only in amplitude and wave length, hence in frequency, 

 which is the reciprocal of wave length. The uniform propagation 

 rate in vacuo is very nearly 300,000 km. per second and the wave 

 lengths so far as ordinarily dealt with range from about .01 to about 

 .0002 mm. For ordinary purposes no attention need be paid to the 

 extremely long wave lengths ranging to .1 mm., to the extremely 

 short ones between .0001 and .0002 mm., or to the enormously shorter 

 one still of the order of magnitude of .0000001 mm. such as the X-rays 

 are believed to be. For the very long waves are not present in material 

 amount in the radiation from ordinary sources. The very short ones 

 are absorbed by a few cm. or dm. of air, and the X-rays are practically 

 only produced in apparatus intended for that purpose. The spectra 

 given by all ordinary sources range between the more modest limits 

 just given. In the earlier literature this spectral range used to be 

 divided into heat rays, light rays, and actinic rays, a distinction 

 wholly artificial since the three effects implied are far from being 

 sharply defined. More generally the whole range is divided into the 

 infra red portion, not ordinarily visible and extending from the longest 

 waves to those of about 760 ixfj., the visible spectrum, extending from 

 about 760 fj-fx to about 395 ix/jl, and the ultra violet portion reaching 

 from 395 nij. to the neighborhood of 200 mm- This distinction is not 

 rigorous or with sharp limits. While artificial distinctions have led 

 to many misunderstandings, all radiation of whatever wave length 

 is convertible into heat when absorbed by material bodies and may 

 produce chemical changes as well. As a matter of fact these latter 

 show a general tendency to increase with the frequency of the oscilla- 

 tions, so that chemical changes are rare in the infra red and increasingly 

 frequent as one approaches the extreme ultra violet. It is this tend- 

 ency that is shown in the pathological changes which may be caused 

 in living cells by the incidence of radiation. 



The rationale of the chemical effect of radiation seems to be that 

 while all radiation transfers energy to the molecules which absorb it 



