NOTES. 



429 



rays, then the particles of polarized light make their vibrations at right 

 angles to the plane of the paper. 



NOTE 904, p. 178. Let A B. fig. 48, be the surface of the reflector, 1C 

 the incident, and CS the reflected rays; then, when the angle SCB is 

 57, and consequently the angle PCS equal to 33, the black spot will 

 be seen at C by an eye at S. 



NOTE 205, p. 179. Let A B, fig. 48, be a reflecting surface. I C the inci- 

 dent, and CS the reflected rays; then, if the surface be plate-glass, the 

 angle SCB must be 57, in order that C S may be polarized. If the sur- 

 face be crown-glass or water, the angle SCB must be 56 55' for the first, 

 and 53 11' for the second, in order to give a polarized ray. 



NOTE 206, p. 180. A polarizing apparatus is represented in fig. 64, 

 where R r is a ray of light falling on a piece of glass r at an angle of 57, 



Fig. 64. 



the reflected ray r a is then polarized, and may be viewed through a piece 

 of tourmaline in 5, or it may be received on another plate of glass, B, 

 whose surface is at right angles to the surface of r. The ray r s is again 

 reflected in a, and comes to the eye in the direction s . The plate of 

 mica, M I, or of any substance that is to be examined, is placed between 

 the points r and s. 



NOTE 207, p. 182. In order to see these figures, the polarized ray r*, 

 fig. 64, must pass through the optic axis of the crystal, which must be 

 held as near as possible to s on one side, and the eye placed as near 

 as possible to s on the other. Fig. 65 shows the image formed by a 

 crystal of Iceland spar which has one optic axis. The colors in the 

 rings are exactly the same with those of Newton's rings given in Note 

 194, and the cross is black. If the spar be turned round its axis, the 

 rings suffer no change; but if the tourmaline through which it is viewed, 

 or the plate of glass B, be turned round, this figure will be seen at the 

 angles 0, 90, 180, and 270 of its revolution. But in the intermediate 

 points, that is, at the angles 43, 135, 225, and 315, another system 

 will appear, such as is represented in fig. 66, where all the colors of the 



Fig. 66. 



