EFFECTS OF RADIANT ENERGY ON THE EYE. 729 



tain other portion goes to heating the electrode or containing tube and 

 the surrounding mechanism. While therefore the spectrum which 

 can be seen or photographed is linear, there is superimposed upon it 

 the continuous spectrum of the radiating solid at rather low tempera- 

 ture and of relatively large extent, since the radiation is not only from 

 the arc or its containing tube but from the immediately heated sur- 

 roundings. Much of the loss of efficiency in both sources referred to 

 comes from this secondary heat radiation. This is particularly the 

 case in the quartz mercury arc of which the actual light-giving effi- 

 ciency is very high, much higher than is indicated even by its really 

 small specific consumption per c. p. 



The existence of this secondary radiation which is mainly of very 

 long wave length, make comparisons between such sources and the 

 ordinary radiating solids very difficult. The following table gives 

 for the magnetite arc the transmission with respect to the total energy 

 of the most important of the various media which we employed, as 

 determined by a Rubens thermopile. 



Absorption of Certain Screens. 



Source Magnetite Arc. 



Filter 

 2 quartz plates each 3 mm. thick 

 Same + 5 cm. distilled water 

 Water cell and Dense Flint Nd 

 " " " Medium Flint No 

 " " Light Flint N^ 

 " " Crown Nb 



Dense Flint N^ 1.69 alone 



Medium Flint N^ 1.63 " 



Light Flint No 1.57 " 



Crown Nb 1.51 " 



It will be observed that the actual transmission of the empty quartz 

 cell consisting of two 3 mm. polished plates was only 53% of the total 

 energy. Of the 47% lost, roughly 15% should be in the reflections 

 from the four surfaces of the two 3 mm. polished plates. The re- 

 mainder, that is more than a third of the total energy, is mainly 



