340 



WELLS'S NATURAL PHILOSOPHY. 



• ^^ If we stand at the junction of two stream3 of water, it will 



iiiterferenooof be noticed tliat when the waves from each meet in the samo 

 darkne^s*?*^"'^^ ^^^^^ °^ vibration, the resulting wave will bo equal to the two 

 combined ; if, however, one wave is half an undulation behind 

 the other, the crest of one will meet the hollow of the other, and compara- 

 tively smooth water will be the result. So if two pencil rays of light, radiat- 

 ing from iwo points, reach a point of interference at the same degree of ele- 

 vation, a spot of double the luminous intensity of either will be produced; 

 but if one is half a vibration behind the other, the result will be, that a dart 

 instead oi a light spot will be apparent. 



How is color The brilliant tints of soap bubbles, and thin 

 tiie'*"inferfe^r- platcs of different transparent bodies, are ex- 

 •nce of light? amples of the interference of light; for the 

 undulations reflected from the first surface interfere with 

 those reflected from the second, and thus produce the 

 various colors. 



The varying play of colors exhibited by films of oil on the surface of water, 

 and the iridescent appearance of mother-of-pearl, the scales of fishes, and the 

 wings of some insects, are all phenomena resulting from the interference of light. 



whatis double 700. Doublc rcfractiou is a property which 

 refraction? certain transparent substances possess, of 

 causing a ray of light in passing through them to undergo 

 two refractions ; that is, the single ray of light is divided 

 into two separate rays. 



Fig. 275. 



Fig. 276. 



A very common mineral called " Iceland spar," 

 which is a crystallized form of carbonate of lime, is 

 a remarkable example of a body possessing double 

 refracting properties. It is usually transparent and 

 colorless, and its crystals, as shown in Fig. 275, havo 

 the geometrical form of a rhomb, or rhomboid; — this 

 term being applied to a solid bounded by parallel 

 faces, inclined to each other at an angle of 105°. 

 The manner in which a crystal of 

 phtnomunon of Iceland spar divides a ray of light in- 

 double refrac- ^q ^^q separate portions is clearly 

 Bhown in Fig. 276 ; in which S T 

 represents a ray of light, falling upon a surface of a 

 crystal of Iceland spar, A D E C, in a perpendicular di- 

 rection. Instead of passing througli without any refrac- 

 tion, as it would in case it had fallen perpendicularly upon 

 the surface of glass, the ray is divided iuto two separate 

 rays, the one, T 0, being in the direction of the original 

 ray, and the other, T E, being bent or refracted. The 

 first of these rays, or the one which follows the ordinary 



