2^2 Dr. Herschei/s Experiments, &c. 



as to the confines of red-coloured light. But their diminishing 

 refrangibility, and increasing power, did not stop here ; for we 

 have pursued them a considerable way beyond the prismatic 

 spectrum, into an invisible state, still exerting their increasing 

 energy, with a decrease of refrangibility up to the maximum of 

 their power; and have also traced them to that state where, 

 though still less refracted, their energy, on account, we may 

 suppose, of their now failing density, decreased pretty fast; 

 after which, the invisible thermometrical spectrum, if I may so 

 call it, soon vanished. 



If this be a true account of solar heat, for the support of 

 which I appeal to my experiments, it remains only for us to 

 admit, that such of the rays of the sun as have the refrangibility 

 of those which are contained in the prismatic spectrum, by the 

 construction of the organs of sight, are admitted, under the 

 appearance of light and colours ; and that the rest, being stopped 

 in the coats and humours of the eye, act upon them, as they 

 are known to do upon all the other parts of our body, by occa- 

 sioning a sensation of heat. 



Slough, near Windsor, 

 March 17, 1800. 



Explanation of Plate XI. 



IN WHICH IS GIVEN A VIEW OF THE APPARATUS. 



A B. The small stand. 

 1, 2, 3. The thermometers upon it. 

 C D. The prism at the window. 



E. The spectrum thrown upon the table, so as to bring the 

 last quarter of an inch of the red colour upon the stand. 



