106 BIOLOGICAL LECTURES. 



our common experience ; and when we meet with such a phe- 

 nomenon as the production of light from the animal tissue 

 without any sensible heat, it appears as if it were a totally 

 isolated physiological phenomenon with no parallel in the 

 ordinary activity of life. 



According to the physicist, however, the external agent 

 which gives rise to the sensation of light in our organism is 

 not much different from that which gives rise to the sensation 

 of heat. Heat and light are only the variations of the same 

 radiant energy. There is an absolute continuity in the nature 

 of the two phenomena the vibrations of ether. 



"When the wave-length is greater than 812 millionths of a 

 millimeter no luminous effect is produced on the eye, though 

 the effect on the thermometer may be very great. When the 

 wave-length is 650 millionths of a millimeter the ray is visible 

 as a red light, and a considerable heating effect is observed. 

 But when the wave-length is 500 millionths of a millimeter, the 

 ray, which is seen as a brilliant green, has much less heating 

 effect than the dark or the red rays, and it is difficult to obtain 

 strong thermal effects with rays of smaller wave-lengths, even 

 when concentrated." l 



The light and heat are so very different to us because we 

 perceive them with different organs of sense. The heat radia- 

 tion, or the waves of ether which have most heating effect, we 

 perceive with the organ of temperature sense, while a similar 

 radiation, with different wave-lengths, which have most lumi- 

 nous effect, we perceive with the organ of sight. 



The difference between heat and light, therefore, " is purely 

 subjective, depending on our organization and not on the nature 

 of external objects." 2 There is an absolute continuity in the 

 nature of external disturbances which create in us the sensa- 

 tions of heat and of light, the difference between them being 

 that of degree and not of kind. Expressed, therefore, in terms 

 of visual sensation, heat is invisible light; and light, expressed 

 in that of temperature sense, is heat with a very little heating 

 effect. 



1 Clerk-Maxwell : Theory of Heat, Chap. XVI., On Radiation, p. 239. 



2 Stokes: On Light, p. 266. 



