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



angle between the axes, and to measure this obtuse angle. As, how- 

 ever, this large angle is usually invisible in air, owing to internal 

 reflection, it is necessary that both angles shall be measured while 

 the sections are immersed in some highly refractive liquid. A simple 

 calculation, involving the two angles measured, enables us then to 

 deduce the true optic axial angle within the crystal. 



The result of a large number of such measurements of optic axial 

 angles has been to show that in all cases where the interference 

 figures are normal, the angle of the rubidium salt is intermediate in 

 value between the angles of separation of the optic axes of the potas- 

 sium and caesium salts of the same triplet. 



We will conclude the lecture by demonstrating to you the beautiful 

 optic axial phenomena in the four abnormal cases to which reference 

 was made when discussing the refraction phenomena, in which for 

 a certain wave-length of light a prism of the crystal only gives one 

 refraction image instead of two. 



You see on the screen the type of interference figure which is 

 given by rubidium sulphate. It is characteristic of the few known 

 interesting salts which exhibit the phenomenon of crossed axial plane 

 dispersion. 



You see next the interference figure afforded by caBsium magnesium 

 selenate. It is characterised by large dispersion, and it will be proved 

 to you in a moment that for blue light this crystal also is apparently 

 uniaxial, and that the figure in white light which you are now con- 

 templating is due to the fact, that for the most luminous colours of 

 the spectrum separation of the optic axes in the horizontal plane 

 occurs. 



It would be interesting to analyse this figure by showing you the 

 curves afforded by the crystal in the pure monochromatic light from 

 the spectroscopic illuminator. But although this can be done most 

 brilliantly for one person at a time looking through the so illuminated 

 observing instrument, there is not light enough for projection. But 

 a series of six photographs, for six specific wave-lengths of light, 

 have been taken with the aid of the apparatus, and you now see them 

 on the screen, each with as exact a reproduction of the colour for which 

 it was taken as possible. The first shows the separation for red lithium 

 light, the second the diminished angle for yellow sodium light, the 

 third the still smaller angle for green thallium light, the fourth the 

 further approach towards the centre for greenish-blue hydrogen light, 

 the fifth shows the uniaxial figure in light of wave-length 466 for 

 which crossing occurs, and the sixth shows the separation in the 

 perpendicular plane for violet hydrogen light. 



We will also show you another six, to prove to you that not only 

 does change of wave-length in the illuminating light provoke extra- 

 ordinary changes in the optic axial angle, but that change of tempe- 

 rature is also provocative of remarkable changes of angle. This set 

 represents the phenomena observed at about 80°. The angle for 



