described, out of all possibility of seeing the object in adirect line.
Those rays which come converging from a dense mediuminto oue more rare, become more convergent; if from ararer medium to a denser, less convergent. So also, diverg-ing rays going out of a dense medium into a rarer, becomemore diverging.
A ray of light, when it enters a different medium, iscalled an incident ray ; and the angle it forms with thesurface, is termed the angle of incidence. While it passesthrough the medium, after being inflected, or diverted fromits former course, it is known as a reflected ray; and theangle it forms with the same surface, is called the angle ofrefraction.
A ray of light entering a dense medium from a rarer one,its velocity only, is supposed to be increased by the force ofattraction, but it moves on in the same line, and is uo'trefracted. If it enters obliquely, its direction becomes lessoblique, to the surface of the medium; or, as is otherwise ex-pressed, is refracted towards the perpendicular; that is,supposing a line drawn perpendicularly to the surface of themedium, through the point which the ray enters, and ex-tended every way, the ray in passing the surface is refractedor bent towards the perpendicular line, or what is the samething, makes a less angle with the perpendicular than whatit did before.
The twinkling of the fixed stars is ascribed to theunequal refraction of light, in consequence of inequalitiesor undulating vibrations in the atmosphere, or of the regionof vaporous particles floating in the air. But this twink-ling has been also ascribed to the immense distance of theorbit of these stars from the terrestrial surface, and to thefaint light they transmit; because the undulation of theatmosphere has not the same effect upon the most distant ofplanets.
The apparent concavity of the sky is only an opticaldelusion, owdng to the incapacity of the organs of vision totake in very large distances; and its blue colour, to a mix-ture of the white light of the sun, blended with the dark-ness occasioned by the boundless extent of space beyondour atmosphere, or perhaps to the construction of air itself;which the ancients supposed to be grey; vide DiodorusSiculus , 7 lib. i. c. 1.