744 VERHOEFF AND BELL. 



from practical illuminants on the human eye is that no sources com- 

 mercially employed for such a purpose are to be regarded as dangerous 

 and that the most ordinary care in providing illumination with which 

 comfortable vision can be obtained is sufficient for complete security 

 against all possibility of injury from radiation. 



Protective Glasses. 



As we have shown, the lens completely screens the retina from 

 abiotic radiations so that attempts further to protect it from such 

 radiations by means of glasses of any kind are superfluous. We have 

 also shown that the retina even of the aphakic eye, imder ordinary 

 conditions is in no danger of injury by any source of light in common 

 use, and is no doubt completely protected by the thick cataract glasses 

 usually worn. In addition we have shown that the retina under 

 ordinary conditions is in no danger of injury from the heat generated 

 within it by the light from such sources. Heat effects are to be feared 

 only in the case of extreme light intensities such as direct sunlight, 

 and, exceptionally, short circuit arcs and lightning flashes. Against 

 these any of the extremely dark glasses are effective. 



As regards the external eye, as just pointed out, it also is in no danger 

 from abiotic radiations from any of the usual light sources. Photoph- 

 thalmia may of course readily be produced by sufficiently long ex- 

 posures at close range to high power arc lights of any kind, or to the 

 quartz or uviol mercury vapor lamps, but it is only under special 

 conditions that such exposures would occur. Here ordinary spectacles 

 of crown glass usually afford sufficient protection, and adequate pro- 

 tection would certainly be afforded by any of the ordinary yellowish, 

 greenish or grayish protective glasses in common use, preferably in 

 the form of coquilles so as to exclude lateral light. These would also 

 afford ample protection against snow blindness or the photophthalmia 

 produced by short circuits. Birch-Hirschfeld ^^ states that from his 

 own personal experience in protracted and regular working with the 

 uviol lamp he found complete protection from photophthalmia with 

 the smoke gray spectacles, and intimates that Stockhausen's claim of 

 having photophthalmia after working with the electric arc lamp, in 

 spite of the fact that he wore ordinary spectacles, is readily explained 

 by the circumstances that common spectacle lenses do not adequately 

 protect the eye from radiation entering laterally. 



For protection of the external eye against extreme heat such as 



