EFFECTS OF RADIANT ENERGY ON THE EYE. 



719 



sources like carbon incandescent lamps and ordinary flames the 

 absorption of the total energy rises to nearly 90%. As we have 

 ah'eady shown this must be increased by miscellaneous losses l)y 

 reflection so that the amount of energy actually available in the 

 image on the retina from such sources is very small. It is quite 

 otherwise with radiants like the sun, which is roughly equivalent 

 to a body of 5,500 to 6,500 degrees absolute as regards the char- 

 acter of its radiation. Froni such a source the specific absorp- 

 tion of water cuts off relatively little, and the total loss of ener'ijy 



.8 .9 1.0 1.1 1.2 1.3 1.1 1.5p, 



Wave length 



Figure 7. Approximate absorption of 5cm water, from data of Aschkinass. 



in the eye is of the order of magnitude of 25 to 30%. In phenomena 

 like eclipse blindness therefore not only is the eye exposed to a 

 very powerful radiating source, but the radiation is of such char- 

 acter that it is not strongly absorbed and hence the energy in the 

 image may rise to very great intensity. The solar radiation curves 

 already shown make it plain that the proportion of energy cut off 

 will be greater the greater the altitude of the sun and the less the 

 general atmospheric absorption. Taking 30% as the total cut off 

 in the eye one may obtain an approximate idea of the energy concen- 

 trated on the retina in observing the sun unscreened, the total radia- 

 tion being about 10^ ergs per square cm. per second; and assuming 

 the pupillary diameter to be about 2 mm., approximately 3% of this 

 energy will enter the eye, and subtracting 30% for absorption and 

 reflection it results that the total energy concentrated in the image 

 would be about 20,000 ergs per second. Taking the area aft'ected 

 as approximately .15 mm. in diameter the concentration of energy 

 in the image is on the basis of nearly 113 X 10^ ergs per square cm. 

 per second. Even if only a quarter or a half of this amount is avail- 

 able in the case of the partially eclipsed sun, it is evident that the 

 immense concentration of energy in the image is sufficient to produce 



