10 On the apparent Changes 



The change of dimensions and of figure, particularly on the 

 verge of the horizon, has exercised the ingenuity of the greatest 

 philosophers, and still remains vexata questio. The change of 

 colours has only been occasionally remarked upon. All these 

 changes will however be found concurrent and illustrative of 

 each other ; whilst tlie various attempts to account for them 

 singly, exhibit such a misapplication of principles, and want 

 of information respecting the ordinary phenomena of light, as 

 will only surprise him who knows not how little is at present 

 generally and correctly understood of light, in its causes and 

 principles of existence. 



Astronomers, who first noticed the changes of place in the 

 heavenly bodies, have variously attempted to account for their 

 existence, and to estimate their amount. They consider the 

 atmosphere as consisting of innumerable concentric laminse, of 

 various densities, and of refractive powers increasing in the 

 approach to the surface of the earth, — that the rays of light, in 

 passing thruogh the atmosphere thus constituted, are subjected 

 to refractions, such as occur at the confines of different refrac- 

 tive media, by which they are similarly bent, in various direc- 

 tions towards the perpendiculars to these laminse, and that 

 changes and elevations of the apparent places of objects are in 

 this manner produced. At the confines of different adjacent 

 refractive media, there is always a division of the light into 

 two portions reflected and refracted, each of them containing 

 a given portion of the incident light. By calculations founded 

 upon the continuance of twilight, after the sun has sunk below 

 the horizon, it has been estimated that at the distance of from 

 40 to 75 miles from the surface of the earth, there are powers 

 in the atmosphere, capable of producing the twilight ; and at 

 these elevations, these concentric laminae are referred to, as 

 reflecting from their supposed lower surfaces, light adequate 

 to the effect produced. 



This estimate, however, of the height of the atmosphere on 

 which twilight depends, is in itself incorrect, as being founded 

 on the false principle that it depends on single reflections at 

 the preceding altitudes of the light of the sun sunk below the 



