i88o.] 
On Water and Air. 
247 
the rays, but the heat falls there far beyond the red. A 
greater amount of heat falls there than falls in any part of 
the visible speCtrum which you see before you. Let me 
suppose that I commence here at this distance. I should 
feel a certain amount of warmth here. I come along , the 
warmth augments. I come farther along, and the warmth 
is greater still. In this position I should have the max- 
imum amount of warmth falling upon my face. Then, as 
I pass on, I come to the visible portions of the spectrum. 
Fig. 24. 
and I find that I am less warm here. I pass into this bril- 
liant red colour, and I still find a less amount of heat falling 
upon me. I go on through the entire speftrum in th s way 
until I reach the extreme blue end, and a gradually de- 
creasing amount of heat falls upon my face. I go even 
beyond this extreme blue end of the speCtrum into the dark 
space beyond it, and allow rays which have no power at all 
to excite vision to fall upon my face, and yet I should be 
able to report to you that far beyond the visible eye a gra- 
dually decreasing warmth was falling upon my face. Thus 
to a blind man this whole range of radiant power would re- 
present itself as heat. Of this vast range along which I 
have gone, the eye, that wondrous organ, seledts a little 
space as available for vision : but beyond this, space we 
have, in both directions, a vast amount of radiation, and so 
strong are the radiations in the invisible portion that those 
radiations beyond the red end have about . seven or eight 
times the radiant power of the whole of the visible speCtrum. 
In this diagram (Fig. 24) the area of that dark mountain (c p) 
represents the power of the invisible radiation to which I 
have referred. The other portion of the diagram represents 
yoL. 11. (third series). T 
