314 On the Velocity. of the calorific tlays 



our sight., yet might be made evident by a careful applica- 

 tion of very minute delicate thermometers, or by any 

 other means suited to discover to us the presence of calo- 

 ric. For thereby that whole portion of the bright imae;e 

 answering to the crescent W would be incapable of heat- 

 ing Or affecting the thermometer, since by hypothesis no 

 rays of caloric are there to be found ; whilst all the rest 

 of the image, as well as the dark space beyond it corre- 

 sponding to the crescent V, would suddenly heat and affect 

 the instruments. 



It may be observed, in passing, that similar extraordinary 

 circumstances would also attend the sun's bright image, in 

 relation to the deoxidizing rays, upon the same assumption 

 of the slowness of their motion and equal refrangibility. 

 For a narrow slip of paper, prepared by the muriate or ni- 

 trate of silver, would not be discoloured when exposed any 

 where within the bright crescent W, which by hypothesis 

 is deprived of the deoxidizing rays ; but would soon be so 

 in any other part of the image, and even when exposed in 

 the dark crescent V. 



But to return to the instance of the calorific rays : it will 

 easily now be understood that the greatest breadth AB, 

 fig. 2, of either crescent W, V, will be more or less consi- 

 derable, according as their velocity falls more or less short 

 of that of the sun's light. For these crescents, so related 

 to the sun's image on the screen, maybe considered merely 

 as creatures of aberration, so to speak, arising from the 

 difference of velocity of the two sorts of rays. AB there- 

 fore, the greatest breadth of either crescent, must in all 

 cases be equal to PO, the separation of the centres belong- 

 ing to the bright image and "circle comprehending the rays 

 of caloric. But, from the principles of aberration, this 

 space PO, or its equal AB, considered, as lying in the 

 plane of the sun's image on the screen, must subtend at 

 the Vertex of the object-glass, an angle equal to the diffe- 

 rence of the aberration of the rays of light and of caloric. 

 If, therefore, we could by any means ever measure this 

 space AB, the greatest breadth of either crescent, we should 

 immediately discover the difference of aberration sought. 

 For, the* focal distance of the image being always given, 

 that would be to AB, suppose found, as radius to the tan- 

 gent of the angle required ; from which the velocity of the 

 calorific ravs is immediately deducible; as will more fully 

 appear in the sequel. 



Further, it is obvious that the space AB, so to be mea- 

 sured in the screen, would become more and more extended 

 3 in 



