WHICH REFLECT LIGHT. 311 



going of the incorrectness of the supposition. The last only of 

 the cases above mentioned remains therefore to be examined, 

 that is, when the refractive index of the masses compared with 

 that of the surrounding medium is assumed to be very nearly = 1 . 



We might here indeed be inclined to seek the cause of the 

 reflexion, not in bodies foreign to the atmosphere, but in the 

 constitution of the air itself, that the latter may be composed of 

 particles separated from each other by empty space, and that 

 the light in its passage from the vacuum to the particle, and vice 

 versa, induces reflexion. Or it might be imagined that the 

 mixture of oxygen and nitrogen, how intimate soever it may 

 appear, is still not perfect enough to form a homogeneous mass, 

 but that, on the contrary, a contirmal passage from oxygen to 

 nitrogen, and vice versa, takes place. On account of the small- 

 ness of the difference between the refractive indices of oxygen 

 and nitrogen, every such passage would only produce a feeble 

 reflexion, but through the innumerable repetitions of the process 

 the total reflexion in the atmosphere might amount to the 

 magnitude observed. 



Without entering upon the probability of any such special 

 assumption, we will simply confine ourselves to the condition, 

 that the reflecting masses possess a very small refractive power. 



To investigate the dispersion produced by these masses, it is 

 necessary, in the first place, to obtain distinct expressions for the 

 dispersive power of the atmosphere ; these expressions will be 

 greatly simplified if we assume that layers of air of the same 

 thickness in all regions of the atmosphere produce equal effects 

 upon the light. This is not the case, in fact, as the air above is 

 less dense than that below; but we may conceive the atmo- 

 sphere to be so compressed that it possesses everywhere the 

 same density as at the surface of the earth, and if the reflecting 

 and dispersive force be supposed to be the same everywhere, 

 both are easily determined by simple constants. 



By the passage of the original parallel sheaf through the at- 

 mosphere it is gradually enfeebled, and expands itself conically. 

 Both effects are really produced suddenly at the forward and 

 hinder surfaces of the layers of air. But as each mass, accord- 

 ing to hypothesis, exerts a very feeble influence, and the effects 

 need manifold repetition to become considerable, we may re- 



