754 



ENERGY AND VISION 



and 0.8 /x, we find 0.343 ergs by unit of surface. In order to de- 

 termine the efficient amount of energy which will affect the eye, 

 it will easily be seen that this quantity will only have to be multiplied 

 by the segment dehmitated on the same chart, by the two ordinates 

 corresponding to the range covered by the slit on the spectrum and 

 the energy distribution curve uncorrected for mirror absorption. 



C 

 o 

 +-> 

 u 



•a 

 G) 



G) 



e 



o 



c 



I 



(0 



o 



^0 

 A 0.4 



0.5 0.55 OB 



0.7 



0.8 yU 



Fig. 3. Energy distribution curve, corrected and uncorrected, in a larger scale 

 in the visible part of the spectrum. A B represents the area covered by a slit 0.25 

 mm. wide. 



Test of the Eyes. 



In order to reduce the intensity of light by a known quantity, 

 a set of absorbing screens was prepared carefully. It was sought 

 to look directly into the beam of Hght instead of using reflected light, 

 in order to avoid the errors arising from the reflection of very faint 

 radiations. By getting screens which could decrease by the same 

 known amount, for example, 90 per cent, the intensity of the incident 

 light, the simple formula 



/ = 7oii:-« 



