316 On the Velocity of the calorific Hays 



though, with this extent of focal distance, we discovered 

 not any symptoms whatever of such crescents, yet still such 

 an apparatus, if sufficiently manageable, would enable us 

 to conclude, in the way of approximation, that the velocity 

 of the calorific ravs comes nearer to that of light than in 

 the ratio above mentioned. For otherwise the crescents 

 would be rendered very palpable by the power of such an 

 apparatus. 



It is almost needless to mention, that the foregoing ex- 

 ample would equally apply in the case of inquiring about 

 the velocity of the deoxidizing rays ; since, by their property 

 of quicklv changing the colour of certain prepared sub- 

 stances, the crescents corresponding to them might, by 

 careful trials, be either made evident, or, if that failed, a 

 certain conclusion might be drawn in the way of approxi^ 

 mat ion. 



It will not probably have escaped notice, that hitherto it 

 has been all along supposed, that crescents only of very small 

 extent would be the consequence of the different aberrations 

 arising from the different velocity of the sun's several radia- 

 tions. This has arisen, in some measure, from having pre- 

 judged the question, bv thinking it most probable that the 

 difference of the velocities may be so inconsiderable. But, 

 for what is at present known, to the contrary, the crescents 

 may be found somewhat more extended, and so much the 

 more favourable for the experiment proposed. 



Possibly it will not immediately occur to those who think 

 that the velocity of the calorific rays may be comparatively 

 exceedingly slow, why we have adhered to the supposition 

 of crescents of any size. It may be said that, according to 

 the principles kid down, and the possible sluggish motion 

 of the rays, we ought, in place of two crescents, rather to 

 look for a total separation of the sun's bright image from 

 the circle comprehending the calorific rays, and to some 

 distance, more or less considerable, on the screen. To this 

 it can be answered, that though the aberration would cer- 

 tainly, in an extreme case, produce that effect, were the 

 velocity so slow as is contended for, vet many facts already 

 well known ineonttstably prove the contrary of that low 

 degree of velocity*. For, in the manifold experiments which 

 have been occasionally made with lenses and mirrors exposed 

 to the sun, some of which were of considerable focal 

 lengths, it was never observed that the burning focus lav 

 separated from the concentrated light, or distant laterally 

 from the sun's bright image ; which ought to be the case 

 on the hypotheses alluded to. No lens or speculum has 



ever 



