294 POPULAR LECTURES AND ADDRESSES. 



heat as light, and the skin of the hand in its 

 perception of radiant heat as heat. We may come 

 to know more ; it may yet appear that there is 

 a continuity. Some of Darwin's sublime specula- 

 tions, may become realities to us ; and we may 

 come to recognise a cultivable retina all over the 

 body. We have not done that yet, but Darwin's 

 grand idea occurs as suggesting that there may 

 be an absolute continuity, between the perception 

 of radiant heat by the retina of the eye and its 

 perception by the tissues and nerves concerned in 

 the mere sense of heat. We must be content in 

 the meantime, however, to make a distinction 

 between the senses of light and heat. And 

 indeed it must be remarked that our sense of heat 

 is not excited by radiant heat only, while it is 

 only and essentially radiant heat that gives to the 

 retina the sense of light. Hold your hand under 

 a red-hot poker in a dark room : you perceive 

 it to be hot solely by its radiant heat, and you 

 see it also by its radiant heat. Now place the 

 hand over it : you feel more of heat. Now, in 

 fact, you perceive its heat in three ways by 



