48 THE NATURE OF ANIMAL LIGHT 



served a more reddish color from luminous tissues of the 

 firefly upon the addition of coagulants such as alcohol, and 

 have noted that the light of Cypridina becomes weaker 

 and more yellow at both low (0°) and high (50°) tem- 

 peratures. The meaning of these color changes w^ill be 

 discussed in Chapter VII. 



The efficiency of any light may be defined in several 

 different ways: (1) By the percentage of visible w^ave- 

 lengths in the total amount of radiation emitted, i.e., 

 visible radiation divided by total (heat, visible, actinic) 

 radiation; (2) by considering, in addition to visible radia- 

 tion ^- total radiation, the sensibility of the eye to differ- 

 ent wave-lengths, visible radiation X visual sensibility -f- 

 total radiation. Visible radiation X visual sensibility is 

 spoken of as luminosity; (3) by the amount of light (ex- 

 pressed in candles) produced in relation to a given expen- 

 diture of energy or in relation to the cost of the energy 

 expended. Thus, of the radiation emitted from an incan- 

 descent electric lamp only a small per cent, is light, the 

 rest being heat and actinic rays. It is therefore very far 

 from being 100 per cent, efficient. If there were no infra- 

 red or ultra-violet in the radiation from an incandescent 

 lamp its efficiency would be 100 per cent, if we disregarded 

 visual sensibility. But if we take into account the fact 

 that the eye is most sensitive to yellow green, a source of 

 light, even though emitting only visible radiation, would 

 not be 100 per cent, efficient unless its maximum of emis- 

 sion corresponded also with the maximum of visual sensi- 

 bility. We shall return to this question in a later para- 

 graph. Looking at the question from the standpoint of 

 energy consumption, the carbon incandescent lamp gives 

 one mean spherical candle for 4.83 watts (watt = 10^ ergs 



