of the Heavenly Bodies. 11 



horizon, whereas it is produced by many and repeated reflec- 

 tions of the vapours floating in the atmosphere, and at last 

 reaches the eye of the spectator by a course so indirect, as to 

 leave no power of estimating the distances from the earth of the 

 reflecting particles. 



Of these floating particles, upon which twilight and other 

 atmospheric phsenomena depend, the altitudes thus assumed 

 exceed all reasonable estimate. All observations respecting 

 the constitution of the air, show twilight to be the product 

 of the whole atmosphere, and its contents near the surface 

 of the earth. The tops of the highest mountains, five miles 

 above the level of the sea, are scarcely attained and sur- 

 mounted by any vapours, by any reflecting particles, capable 

 of producing sensible effects from their sizes or numbers. The 

 supposed height of the atmosphere capable of reflecting the 

 height of twilight, being 50 miles according to Keill, whose 

 estimate is a mean between others, the lengths of air traversed 

 by light in the horizon is 12 times greater than in the zenith. 

 (Vide KeilVs Astronomy , p. 244.) This gives the enormous 

 tract of 600 miles for the passage of the horizontal light of the 

 luminaries. Through an atmosphere of 75 miles, 50 miles, or 

 40 miles height, divided into various reflecting and refracting 

 laminee, as alleged, not the light of twilight, not the light of 

 the meridian sun would reach the surface. 



Euler supposed a single bending of the rays at the confines of 

 «ether and atmospheric air, such as occurs between any other 

 two transparent refractive media. This hypothesis escapes the 

 preceding objection, but is subjected to the difficulties of an 

 hypothesis which would establish confines between air and 

 fiether, similar to those between other transparent media of dif- 

 ferent refractive powers, together with all tlie other objections 

 to atmospheric refractions. 



The atmosphere is not a laminated but a continuous body. 



In the passage of light through a transparent continuous 

 medium, there is neither reflection nor refraction by the 

 medium ; and therefore there are in fact no atmospheric refrac- 



