22 On certain Conditions under xdiich 



It has often struck me that there is a circumstance, appa- 

 rently of moment, affecting the Hght imparted to us by the 

 heavenly bodies, which is neglected by all inquirei's, and of 

 which, so far as I am aware, no sufficient notice has yet 

 been taken. It is usual to reason as if the whole light sup- 

 plied by each of those bodies were given out under the same 

 identical conditions, as if from a mathematical point or centre ; 

 though a very little reflection must convince us that this is 

 far from being either strictly, or even practically, the case ; and 

 the circumstance, therefore, to which I allude is the actual 

 form and dimensions of those luminaries, conspiring to pro- 

 duce results, which, although they may be difficult at first 

 sight to point out and explain, can scarcely be without their 

 influence on some of the phaenomena exhibited by that great 

 natural agent. 



It is obvious that, whatever theory be true with respect to 

 the constitution of light, whether the Newtonian, the undula- 

 tory, or any other, the rays which reach us from a luminous 

 body are propagated by a very complicated process from all 

 parts of its surface; and although in straight lines always, 

 yet in all directions outwards from each point in that surface. 

 It is difficult for the mind to form any conception of the in- 

 finitely intricate system of radiation which is thus set in action. 

 The instance so often given to illustrate the theory of undu- 

 lations, of a stone dropped into a pool of water, is wholly in- 

 adequate to represent it. If a perfect sphere were allowed 

 to fall into a fluid perfectly at rest, the wave caused by its dis- 

 placement of the fluid would be impelled by a force acting 

 simply in straight lines perpendicular to the surface of the 

 sphere, in the plane of its horizontal great circle, which would 

 be that of its maximum displacement; and therefore the force 

 by which the wave would be driven upon any given point 

 could only be exerted in a straight line joining that point and 

 the centre of the sphere. If this were the case with the wave 

 of light, the eye would see that point alone of the radiating 

 surface through which such a line would pass ; and as it does, 

 in fact, receive the impression of light from every point upon 

 it from whence a straight line can be drawn to the eye, it 

 follows that the motion of light (whatever be its nature) is 

 propagated by a system of infinite divergence at every con- 

 ceivable angle from every portion of that surface. 



If we now look at the bearing of these considerations upon 

 the point to which we alluded at the outset, we must remark 

 that the heavenly bodies are for the most part spheres, or 

 spheroids, of enormous size ; and under the small angle at 

 whidi we view them we may be said to receive light from one 



