which proceed from the Sun, 319 



limb keeps on one of them as it passes through the field of 

 view. Then will the other wire lie in the plane of the eclip- 

 tic, and so will become the directrix, marking out the ver- 

 tex of each crescent when the axis of the speculum is again 

 pointed to the centre of the disk. 1 



Some have imagined that rays of any kind which are less 

 refrangible than others must, on that account, move with 

 a greater velocity before incidence. According to this opi- 

 nion, those calorific rays, which Dr. HerscheT's late admi- 

 rable discovery has shown to be less refrangible than light, 

 must move the swiftest of the two ; and light, in like man- 

 ner, swifter than the deoxidizing rays, whieh, by a more 

 recent discovery, highly interesting also, have been found 

 more refrangible. But such conclusions do not appear to 

 be at all sufficiently supported. They depend on several 

 assumptions ; particularly on this, that the constituent par- 

 ticles of the three different kinds of rays, were they to begin 

 their motion at equal distances from the refracting surface, 

 would be equally accelerated in. the perpendicular direction 

 by the power of the medium. For what can be shown to 

 the contrary, however, this may be wide of the truth. May 

 not the refractive power of the same medium accelerate very 

 differently the particles of which rays wholly different in 

 their nature are constituted ? May not those even be re- 

 fracted the least whose velocity is the slowest, and m a ratio 

 little depending on the velocities ; according to what may 

 be called elective attractions, or peculiar affinities, whieh 

 may obtain between the medium and rays differing so much 

 in kind ? The phenomena of the different dispersions of 

 the constituent parts of the same rays, namely light, evi- 

 dently show some variable affinity of this sort, according to 

 the nature of the refracting medium. Why, therefore, 

 may not such an affinity be anomalous, according to the 

 nature of the rays which differ from one another so much } 



Though therefore the rays of caloric, as emitted *by the 

 gun, were all found the least refrangible, yet their velo- 

 city may be far inferior to that of light. For what we know, 

 the fact really may be, that the retracting medium governs 

 the motion of the latter much more powerfully than that of 

 the calorific rays. 



As in the present state of our knowledge we can entertain 

 nothing but conjecture on a point so interesting, so the ne- 

 cessity of having recourse to some experimental mode of 

 proof, to clear away doubts and lead us to the truth, appears 

 in a strong point of view, 



Before 



