388 Dynamic Theory. 



light and heat as they come to us from the sun, constitutes the most 

 powerful dynamic agency. Although it is only motion, it is motion of 

 a real physical body, which is proved by the fact that it is influenced by 

 the bodies it comes into contact with, and influences them in return. It 

 is necessary to study these things in order to understand the manner and 

 extent of the influence they have had in building up organisms, includ- 

 ing ourselves, and keeping them in operation after they are built up. 

 In this chapter there will be recalled some of the effects which are pro- 

 duced in light and heat by their encounter with ponderable bodies. 



Law of Refraction, also called the law of Snellius. It was stated 

 above that a beam of light is refracted when it passes from a rare to a 

 denser medium, as from air to water. 



P Q, Fig. 161, is a normal or 

 perpendicular to a reflecting and re- 

 fracting surface, M N. A C P, 

 angle of incidence equal to P C J, 

 the angle of reflection. The law of 

 -Snellius is; that for the same refracting 

 substance, the eye being supposed at 

 A and the object seen really at D, 



j^r u. JY ^ but appearing to be at B, the line C 



7 I^Q^^ B bears a constant ratio to C D, 



/ / without regard to the inclination of 



the line A C to the refracting sur- 

 face M N. Likewise the secants 

 G- and C H are in the same ratio, and 

 so are the sines of the respective an- 

 gles A X and D L. But for differ- 

 FIG. 161. ent substances the angle A C H is 



different according to density when other things are equal. The con- 

 stant ratio found by dividing AX by D L is, in the refraction between 

 air and water 1^ or 1.333; between air and glass it is from about 

 1.50 to 1.53 according to density ; between water and glass about 1.14. 

 ( The index for red light is greater than that for violet, the former being 

 1.346, air to water, and the latter 1.333 ; this accounts for the colors 

 of the rainbow. ) The index for diamond is 2.5 ; for lead chromate 2.97. 

 Inflection or Diffraction. < 'If any very small object be placed in a pencil 

 of divergent light admitted through a minute aperture into a dark room, 

 its shadow will appear materially larger than it ought if light passes its 

 edges in straight lines, and moreover, any opaque object, large or small, 

 exhibits along the edges of its shadow a border of at least three distinctly 

 tinted fringes, the brightest and broadest of which is next the shadow. ' ?1 

 1 Prof, Barnard in Smithsonian Reports. 



