which proceed from the Sun. 317 



ever been found to produce a bright and cold image of the 

 sun in one place, and to burn at another. This fact is now 

 referred to, for the first time so far as we know, as a com- 

 plete demonstration that the motion of the calorific rays 

 looks up to some high scale of rapidity ; though as yet we 

 know not whether it equals or exceeds, or how far it falls 

 short of, the velocity of light. This view of the subject will 

 fully account for our having confined the foregoing illus- 

 trations to the case of even small crescents. 



Part IIL 



In order to take still a nearer practical view of the whole 

 of this matter, it is full time now to mention, that though, ' 

 for the sake of greater simplicity, all the illustrations have 

 proceeded on the supposition of the sun's ima^e being 

 formed by refraction, yet the image made by reflection from. 

 a metallic speculum would exhibit the same phaenomena 

 of the crescents, &c. as have been treated of at so much 

 |ength. 



Indeed it is only in this way that trials on the principles 

 advanced can be attempted : and for two reasons. 1st, Be- 

 cause we can easily command a competent aperture for the 

 object speculum, in order to obtain, at a long focal distance, 

 an image of the sun sufficiently hot for the intended experi- 

 ments. Double or triple the force of common sunshine 

 may probably be found the most commodious. 2dlv, Be- 

 cause, by reflection, the foci of the calorific and deoxidizing 

 rays, how much soever they differ from light in refrangi- 

 bility, would lie in the same planewith the luminous image. 

 This condition, it is evident, would be quite necessary for 

 discovering the crescents, if such existed to any sensible 

 extent*, and especially for measuring AB, their greatest 

 breadth. In order to this, it would not be requisite to have 

 the sun's image projected on the under side of any screen 

 placed at the focus of the speculum, where it could not 

 be seen or examined without uneasy postures and much in- 

 convenience. The place of the image, free from any screen, 

 and when formed in the air, might in several ways be made 

 visible so far as to know where to apply the thermometers 

 or minute spherules of wax or jelly at the two opposite 

 points, where the crescents, if existing, ought to be broad- 

 est, as at A and B, which may be called their vertices. 



As the success of the experiments would much depend 

 upon our knowing at any time the precise place of either 

 vertex, the following short theory of them ought to be fully 

 understood :-. — 



When 



