THE SIX GA TE WA YS OF KNO WLEDGE. 293 



vision, but still in virtue of radiant-heat, a hot 

 body ; if illuminated by light, or if hot enough to 

 be self-luminous, red-hot or white-hot, you see it : 

 you can both see a hot body, and perceive it 

 by its heat, otherwise than by seeing it. Take 

 a piece of red-hot cinder with the tongs, or a red- 

 hot poker, and study it ; carry it into a dark room, 

 and look at it. You see it for a certain time ; 

 after a certain time you cease to see it, but you 

 still perceive radiant heat from it. Well now, 

 there is radiant heat perceived by the eye and 

 the face and the hands all the time ; but it is 

 perceived only by the sense of temperature, when 

 the hot body ceases to be red-hot. There is then, 

 to our senses, an absolute distinction in modes 

 of perception between that which is continuous 

 in the external nature of the thing, namely, radiant 

 heat in its visible and invisible varieties. It 

 operates upon our senses in a way that I cannot 

 ask anatomists to admit to be one and the same 

 in both cases. They cannot now at all events, 

 say that there is an absolute continuity between 

 the retina of the eye in its perception of radiant 



