390 Dynamic Theory. 



impression of this shell in black wax will have a like striated surface 

 and will likewise reflect certain colors, the rest being destroyed by interfer- 

 ence. Interference in such cases is caused by the reflection of some of 

 the light from the sides of the fine furrows or troughs in such a way 

 that it loses position to the extent of a half of its wave-length, and thus 

 abolishes other rays of the same color not so altered. The colored rings 

 of Newton are formed by a double convex lens pressed against the plane 

 side of a piano convex lens. Light reflected from the latter lens will 



A " * AUi ^!!WK^\W^ FIG. 162. Showing rings of Interference. 



U?MM7/7?s)WMWr))s/swsws* A. Piano convex lens on flat surface. 

 B.-Ptan of colored rings formed by A. 



take the form of colored rings alternating with 

 dark ones. They occupy a certain mathemati- 

 cal relationship with one another, the squares 

 of the diameters of the colored rings being to 

 FIG. 162. each other in the ratio of 1, 3, 5, 7, &c., and 



the squares of the diameters of the dark rings are as 2, 4, 6, 8, &c. 

 Beyond the seventh ring the colored and dark rings are so crowded to- 

 gether as to become practically fused into one, white light resulting. 

 The colors of the rings beginning at the center are: 1, black, blue, 

 white, yellow, red ; 2, violet, blue, green, yellow, red ; 3, purple, blue, 

 green, yellow, red ; 4, green, red ; 5, greenish blue, red ; 6, greenish 

 blue, pale red ; 7, greenish blue, reddish white. This phenomenon is 

 explained as the interferences between rays which are reflected from the 

 first surface encountered by the incident beam ; and those which pass 

 through to the second lens and are reflected thence. 



Double refraction. A beam of light passing through a crystal not 

 homogeneous is divided, one ray being refracted according to the law of 

 Snellius and called from that the ordinary ray, and the other is called 

 the extraordinary ray. This is notably true in the case of the crystals 



of Iceland Spar. These crys- 

 tals are large and are rhombo- 

 hedral in shape ; see fig. 163. 

 The angles of the Rhomboidal 

 face A, B, C, D are A and C 

 101 55' and D and B 78 05' 

 FIG. les.-Crystal of Iceland Sar. respectively. The angles of in- 



clination D F E and C E F are 74 55 ' and 105 05'. The shortest 

 diagonal A E is the crystallographic axis. 



If the angles A and E be truncated perpendicularly to the crystallo- 

 graphic axis, light passing through the crystal parallel with this axis 

 will not be doubly refracted. Any such line is also called the optic 

 axis. . Any plane parallel with the optic axis is called a principal plane 

 or principal section. Any plane at right angles to the optic axis may 



