738 VERHOEFF AND BELL. 



First, by the experiments which have been heretofore described we 

 have made it clear that there can be no injury done to the retina by 

 ultra violet light as such, even the most severe exposures failing 

 completely to produce any effect whatever. Second, all thermic 

 effects of energy from any source on the external eye are at once 

 ruled out of consideration by the immediate discomfort produced by 

 excessive heat. No person would tolerate extreme heat radiation on 

 the external eye for a period long enough to produce the slightest 

 damage. There remain therefore to be considered thermic effects 

 within the eye, and specially those due to the focussing of intense 

 radiation upon the retina as in eclipse blindness; over stimulation of 

 the physiological processes in the retina, that is pathological effects 

 due to light as such in its action on the retinal structure, and finally 

 those abiotic effects of the extreme ultra violet rays on the external 

 eye properly known as photophthalmia. It has been our purpose to 

 ascertain the practical risks incurred in the use of artificial illuminants 

 and the precautions required to avoid danger. To this end have we 

 sought especially the quantitative relations in the action of radiant 

 energy upon the eye. We have, therefore, experimented with the 

 most powerful sources used for practical lighting under conditions of 

 intensity immensely greater than occur in their every day use. We 

 have shown (page 728) that as regards the general thermic effects of 

 energy upon the eye there is no chance of damage to the retina or to 

 the media of the eye under any practical conditions of use. With 

 respect to damage to the retina in particular we have been unable to 

 produce it except by exposures and intensities enormously greater 

 than could possibly be reached in the use of artificial sources of light. 

 We have used beside the quartz mercury arc which is not particularly 

 strong in general radiation, a 750 watt magnetite arc which takes the 

 greatest amount of energy of any arc light ordinarily used for illumi- 

 nating purposes and the 750 watt nitrogen lamp which gives more 

 ultra violet than any other incandescent source, and which failed 

 completely to produce any specific damage to the eye, although in one 

 experiment the animal was overcome by the general heat effect, as in 

 sunstroke. This occurred after an exposure of 1| hours at 20 cm. 

 from the filament. A second experiment with an exposure of 2 hours 

 at the same distance, in which the animal was well protected from the 

 heat and the head kept cooled with water, showed no damage to the 

 eye of any kind. This source as a whole focusses less sharply than 

 does the arc lamp taking an equal amount of energy, and in this case 

 we have already shown that the retina would not be subject to damage 



