140 Mr. A. E. Tutton [May 2, 



ellipsoid may therefore be regarded as free to rotate about this axis, 

 for the inclination of the other two rectangular axes of the ellipsoid, 

 which lie in the plane of symmetry, may be any whatsoever with 

 respect to the two inclined morphological axes which lie in that 

 plane. 



Within the three models of sections of our rhombic sulphate 

 crystals you now see three illuminated ellipses. They represent the 

 sections of the three optical ellipsoids ; but in this case the inner one 

 corresponds to the caBsium salt, and represents the ellipsoidal line 

 to which light waves emanating from the imaginary centre of the 

 crystal would penetrate in a given interval of time. The middle one 

 shows the distance to which they would penetrate if the crystal were 

 one of rubidium sulphate or selenate ; and any point on the outer 

 one, which is much nearer to the middle one than the outer one is, 

 represents the position at which the light waves would arrive in the 

 same interval of time if the crystal were one of potassium sulphate 

 or selenate. 



The velocity with which light travels through the crystals of the 

 three salts is thus observed to vary in the same manner as does the 

 atomic weight of the alkali metal present in the salt. 



In the case of the monoclinic double sulphates and selenates, we 

 have a further phenomenon of even greater interest exhibited. For 

 not only does the velocity along the three rectangular axes of the 

 ellipsoid vary with the atomic weight, but the possible rotation of 

 the whole ellipsoid about the symmetry axis is found to actually 

 occur ; and to occur, moreover, to a very considerable extent, some- 

 times amounting to as much as 20°. 



Further, most interesting of all, the rotation varies in a perfectly 

 regular manner throughout all the triplets, in accordance with the 

 atomic weight of the alkali metal present in the salt. This may 

 be demonstrated to you by means of the lantern slide which you now 

 see projected on the screen. You observe the outline of a crystal, 

 arranged so that the screen represents the symmetry plane, and 

 within it an ellipsoid. The latter is at present arranged as it is 

 usually situated in any potassium salt of the series, its major axis 

 being inclined so that its top is somewhat to the left of the vertical 

 morphological axis. The ellipsoid will now be rotated to the 

 approximate position, further on the left of the vertical axis, which 

 it occupies in any rubidium salt of the series, by means of a simple 

 mechanical device ; and again it is rotated much further, to about 

 the situation which it takes up in any caesium salt. 



In this phenomenon the accelerating progression according to the 

 atomic weight of the alkali metal is beautifully exhibited. 



It may now interest you to learn how these results have been 

 obtained. We have to determine, first the position of the optical 

 ellipsoid, and next its dimensions. That is to say, we have to 

 determine the velocity of light transmission in all the various direc- 



