333 WELLS'S NATURAL PHILOSOPHY, 



son they are the first to appear in the morning when tlie 

 sun rises, and impart to the morning clouds red or crim- 

 son colors. 



Let us suppose, as in Fig. 274, a ray of light proceeding from the sun, S, 

 to enter the earth's atmosphere at the point P. The red rays, which com- 

 pose in part the solar beam, being the least refrangible, or the least deviated 

 from their course, will rea'/h the eye of a spectator at the point A ; while 

 the yellow and blue rays, being refracted to a great^ir degree, will reach tho 

 surface of the earth at the intermediate points B and C. They will, conse- 

 quently, be quite invisible from tlie point A. 



Tlie red and golden appearance of the clouds at morning and evening is 

 aL?o due in part to the fact, that aqueous vapor on the point of being con- 

 densed, only allows the red and yellow rays of light to pass through it. For 

 this reason, if the sun be viewed through a column of steam escaping from 

 a boiler, it appears of a deep red, or crimson color. The same thing may be 

 noticed during a drought in summer, when the air is filled with dry exhala- 

 tions. 



■What is 697. The irregular brilliancy of the stars, 



Twiakimgr known as twinkling, is supposed to be due to 

 unequal reflections vf light occasioned by inequalities and 

 undulations in the atmosphere. 



Tio^ i3 color 698. Light, according to the undulatory 

 the unduhitory thcorv, is occasioucd by the vibrations or un- 

 theoryofiisht? Julatious of a certain clastic medium diffused 

 throughout all space, called Ether. Color, according to 

 this theory, depends on the number of vibrations which 

 are made in a certain time ; those vibrations which are the 

 most rapid, producing upon the eye the sensation of violet, 

 and those which are the slowest, the sensation of red. 



The analogy between sound and light, according to the 

 ^Ihere Ikn ^mdulatory theory, is perfect, even in its minutest circum- 

 tween color and stances. When a certain number of vibrations of a musical 

 music? chord are caused in a given time, we produce a required 



sound ; as the vibrations of the chord vary from a quick to a 

 slow rate, we produce sounds sharp or grave. So with light ; if the rate at 

 which the ray undulates is altered, a difierent sensation is made upon the 

 organs of vision. 



The number of aerial vibrations per second required to produce any particu- 

 lar note in music has been accurately calculated, and it is also known that 

 the ear is able to detect vibrations producing sound, through a range com- 

 mencing with 15, and reaching as far as 48,000 in a second. So also in tho 

 case of light, tho frequency of vibrations of the other required for the produc- 



