NOTES. 



471 



the reflected ray r s is them polarised, and may be viewed through a piece 

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



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

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

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

 the points r and s. 



NOTE 212, p. 187. In order to see these figures, the polarised ray rs, 

 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 colours in the rings are 

 exactly the same with those of Newton's rings given in Note 199, 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, 

 108, and 270 of its revolution. But in the intermediate points, that is, 

 at the angles 45, 135, 225, and 315, another system will appear, 

 such as represented in fig. 66, where all the colours of the rings are com- 

 plementary to those of fig. 65, and the cross is white. The two systems 

 of rings, if superposed, would produce white light. 



Fig. 65. 



Fig. 66. 



NOTE 213, p. 188. Saltpetre, or nitre, crystallises in six-sided prisms 

 having two optic axes inclined to one another at an angle of 5. A slice 

 of this substance about the 6th or 8th of an inch thick, cut perpendicularly 

 to the axis of the prism, and placed very near to s, fig. 64, so that the 



