1902. J en the Constitution of CrijstaU. 141 



tions in the crystal. Determinations of the refractive index in different 

 directions will afford us the necessary data, and if we can discover 

 the directions of maximum and minimum velocity we shall obtain 

 all that we require if we make the determinations of refractive index 

 for these two directions and for a third direction at right angles to 

 the plane containing them. For these three directions will be those 

 of the three axes of the ellipsoid. 



It will next be demonstrated to you how we discover the positions 

 of the axes of the optical ellipsoid. 



It is scarcely necessary to introduce to a Royal Institution 

 audience the magnificent pair of Nicol prisms which formerly be- 

 longed to the late Mr. Spottiswoode. The analysing Nicol is at 

 present arranged with its vibrating direction parallel to that of the 

 polarising prism, and to the horizontal cross-wire which you see 

 appearing on the screen, so that light passes to the screen. 



There is also focussed the outline of a section of a monoclinic 

 crystal, placed between the two Nicols, and which you may take as 

 representing a crystal of one of our double sulphates or selenates ; 

 for it behaves precisely similarly, and is immensely larger than any 

 crystal that could be obtained of one of those salts. We now rotate 

 the analyser so that its vibrating direction is at right angles to that 

 of the polariser, when you see the crystal section brilliantly coloured 

 on the dark field. 



If, however, we rotate the crystal section in its own plane, you 

 observe that the coloured light becomes weaker, until, at a certain 

 position, it is altogether extinguished. Further rotation causes light 

 to again appear, and if we complete the circle of rotation we shall 

 find that the crystal becomes four times dark and four times light. 

 That is, there are two directions, at right angles to each other, in 

 which extinction occurs. These two directions are those of two axes 

 of the optical ellipsoid. If the crystal is so prepared as to show on 

 its edge traces of one or two natural faces, and if our rotating stage 

 is divided, it is quite easy to determine the angle between any given 

 trace of a face and either of the extinction directions. In the case of 

 monoclinic crystals the section is cut parallel to the symmetry plane, 

 and the reference edge will usually be a trace of one of the faces in 

 the primary zone perpendicular to the symmetry plane. That is the 

 case with the two faces whose traces you see are left after the grind- 

 ing of the large section you observe on the screen. A much more 

 refined method of measuring the angle between the extinction direc- 

 tion and the basal plane, based on this principle, was employed in 

 the research. 



We will next illustrate the determination of the three refractive 

 indices, corresponding to the three velocities along the three axes of 

 the ellipsoid. There is here a large 60°-prism of a crystal which 

 behaves similarly to the salts we are discussing, and which has been 

 specially cut and polished for this lecture by Mr. Hilger. The re- 



