500 NOTES. 



NOTE 201, p. 204. Both of the parallel rays O o and E o, fig. 63, are polarised 

 on leaving the doubly refracting crystal, and in both the particles of light 

 make their vibrations at right angles to the lines O o, E o. In the one, how- 

 ever, these vibrations lie, for example, in the plane of the horizon, while the 

 vibrations of the other lie in the vertical plane perpendicular to the horizon. 



NOTE 202, p. 204. If light be made to fall in various directions on the 

 natural faces of a crystal of Iceland spar, or on faces cut and polished arti- 

 ficially, one direction A X, fig. 63, will be found 1 , along which the light passes 

 without being separated into two pencils. A X is the optic axis. In some 

 substances there are two optic axes forming an angle with each other. The 

 optic axis is not a fixed line, it only has a fixed direction ; for if a crystal of 

 Iceland spar be divided into smaller crystals, each will have its optic axis ; but 

 if all these pieces be put together again, their optic axes will be parallel to 

 A X. Every line, therefore, within the crystal parallel to A X is an optic axis; 

 but as these lines have all the same direction, the crystal is still said to have 

 but one optic axis. 



NOTE 203, p. 206. If I C, fig. 48, be the incident and C S the reflected rays, 

 then the particles of polarised light make their vibrations at right angles to 

 the plane of the paper. 



NOTE 204, p. 206. Let A B, fig. 48, be the surface of the reflector, I C the 

 incident and C S the reflected rays ; then, when the angle S C B 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. 207. Let A B, fig. 48, be a reflecting surface, I C the incident 

 and C S the reflected rays ; then, if the surface be plate-glass, the angle S C B 

 must be 57, in order that C S may be polarised. If the surface be crown-glass 

 or water, the angle SC B must be 56 55' for the first, and 53 11' for the 

 second, in order to give a polarised ray. v 



NOTE 206, p. 209. A polarising 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, the re- 



fleeted ray r s is then polarised, and may be viewed through a piece of tourma- 

 line in a, 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 *, and comes 

 to the eye in the direction s E. 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. 211. In order to see these figures, the polarised ray r s, fig. 64, 

 must pass through the optic axis of the crystal, whch must be held as near as 

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



