NOTES. 



491 



NOTE 181, p. 163. The long cross-lines of fig. 46 show the two systems 

 of nodal lines given by M. Savart's lamina;. 



Fig. 46. 



J 



I I I I I 



TT 



I I I I I 



NOTE 182, p. 163. The short lines on fig. 46 show the positions of the nodal 

 lines on the other sides of the same laminae. 



NOTE 183, p. 164. Fig. 47 gives the nodal lines on a cylinder, 

 paper rings that mark the quiescent points. 



Fig- 47. 



nth the 





NOTE 184, pp. 154, 171, 172, 175. Reflection and Refraction. Let P C p, 



Fig. 48. 



48, be perpendicular to a surface of glass 

 or water A B. When a ray of light, 

 passing through the air, falls on this 

 surface in any direction I C, part of it 

 is reflected in the direction C S, and 

 the other part is bent at C, and passes 

 through the glass or water in the di- 

 rection C R. I C is called the incident 

 ray, and I C P the angle of incidence; A 

 C S is the reflected ray, and P C S the 

 angle of reflection; C R is the refracted 

 ray, and p C R the angle of refraction. 

 The plane passing through S C and I C 

 is the plane of reflection, and the plane 

 passing through 1 C and C R is the plane 

 of refraction. In ordinary cases, C I, 

 C S, C R, are all in the same plane. We see the surface by means of the re- 

 flected light, which would otherwise be invisible. Whatever the reflecting 

 surface may be, and however obliquely the light may fall upon it, the angle of 

 reflection is always equal to the angle of incidence. Thus I C, P C, being 

 rays incident on the surface at C, they will be reflected into C S, C S', so that 

 the angle S C P will be equal to the angle I C P, and S' C P equal to I' C P. 

 That is by no means the case with the refracted rays. The incident rays I C, 

 I' C, are bent at C, towards the perpendicular, in the direction C R, C R 7 ; and 

 the law of refraction is such, that the sine of the angle of incidence has a con- 

 stant ratio to the sine of the angle of refraction: that is to say, the number ex- 

 pressing the length of I m, the sine of I C P, divided by the number expressing 

 the length of R n, the sine of R C p, is the same for all the rays of light that 



