EFFECTS OF RADIANT ENERGY ON THE EYE. 775 



and still cut down the visible spectrum but little — 5% for a thickness 

 of 10 mm. They found that fluorescence of the rabbit's lens was not 

 diminished by 18 mm. of plate glass, therefore rays less than 300 nfx, 

 could not be held responsible for the fluorescence. Nor did flint glass 

 absorbing to 350 /xix decrease the phenomenon but when Euphos glas 

 was interposed the fluorescence was stopped. Therefore they limited 

 the range of fluorescing rays to 350 mm to 400 nix. If they allowed the 

 light to traverse both a blue uviol glass and a Euphos glas before 

 striking the rabbit's eye there was no fluorescence of the lens. Now 

 when the Euphos glas only was removed after adaptation had taken 

 place, much lid-spasm and blinking of the rabbit's eyes took place as 

 the fluorescence began. They laid great stress on this occurrence as 

 an indication of the painful and injurious effect of this group of rays, 

 350 MM to 400 niJL, on the retina. From a study of the spectrum in 

 this region photographed through the whole eye media and from the 

 appearance of the fluorescence itself, they came at this time to the 

 conclusion that fluorescence was due not as Helmholtz said to trans- 

 formation of short wave ultra violet light to longer waves of different 

 length in the visible field, but due to the appearance of a new spectral 

 color lavender-gray of definite wave length. Not only did they con- 

 sider fiuorescence, as did Widmark, to decrease with age but also with 

 length of time of exposure, since the fresh lens from a gliomic eye 

 of a child diminished notably in fluorescing power after exposure of a 

 few hours. This decrease of fluorescing power they attributed to 

 some breaking down or change in chemical composition which they 

 supposed, without analytical proof, to be involved in the production 

 of fluorescence. 



Birch-Hirschfeld ^^ at once objected to these conclusions, consider- 

 ing that nothing had been offered as real proof against the Helmholtz 

 theory of fluorescence. He maintained that nothing had been shown 

 as to the nature of fluorescence and even questioned whether the 

 range of rays responsible could be established in the manner described. 

 The fact that lid-spasm was elicited as described he could not consider 

 as evidence of a retinal injury, since it is well known that such reflexes 

 are easily produced by harmless light on the dark adapted eye. Aside, 

 then, from injury to the retina by the range of rays mentioned, a mere 

 change of intensity would account for the lid-spasm since Helmholtz 

 has shown that fluorescent light is many times as intense, physiologi- 

 cally, as the light producing it. As to the diminution of fluorescence 

 with age Birch-Hirschfeld considered the question still open, since he 

 found the fluorescing power of the lens of an individual 70 years old 



