of the invifible Rays of the Sun, 1^ 



from the fun, confifts of rays that are differently refrangible. 

 The range of their extent, when difperfed by a prifm, begins 

 at violet-coloured light, where they are moft refracled, and 

 have the leaft efficacy. We have traced thefe calorific rays 

 throughout the whole extent of the prifmatic fpe&rum, and 

 found their power increafing, while their refrangibility was 

 lefiened, as far as to the confines of red-coloured light. But 

 their diminifhing refrangibility, and increafing power, did not 

 flop here ; for we have purfued them a confiderable way be- 

 yond the prifmatic fpedrum, into an invifible ftate, ftill ex- 

 erting their increafing energy, with a decreafe of refrangi- 

 bility, up to the maximum of their power; and have alfo 

 traced them to that ftate where, though ftill lefs refracted, 

 their energy, on account, we may fuppofe, of their now fail- 

 ing denfity, decreafed pretty fail; after which, the invifible 

 therniometrical fpeclrum, if I may fo call it, foon vanifhed. 



If this be a true account of folar heat, for the fupport of 

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

 admit, that fuch of the rays of the fun as have the refrangi- 

 bility of thofe which are contained in the prifmatic fpe&rum, 

 by the conftru&ion of the organs of fight, are admitted under 

 the appearance of light and colours ; and that the reft, being 

 flopped in the coats and humours of the eye, act upon them, 

 as they are known to do upon all the other parts 0/ our bod^, 

 by occafioning a fenfation of heat. 

 Slough, near VVindfor, 



March 17, 1800. 



Explanation of Plate I. fig. 2. 



AB, the fmall (land. 1,2,3, tne thermometers upon it, 

 CD, the prifm at the window. E, the fpe&rum, thrown 

 upon the table fo as to bring the laft quarter of an inch of 

 the red colour upon the Rand, 



III, £*- 



