386 Royal Society. 



passing through the axis ; the other tint appears in every azimuth in 

 the directions perpendicular to the axis, and it is polarized in a plane 

 perpendicular to the axis. The latter of these colours does not 

 appear at all, if the crystal is examined in the direction of the axis ; 

 if it depend at all on transverse vibrations, all vibrations of this kind, 

 transverse or perpendicular to the axis, are at once excluded, and the 

 only vibrations that can possibly belong to the colour of the extra- 

 ordinary raj- produced in the crystal, are those parallel to the direc- 

 tion of the axis. But agreeably to observation the plane of polariza- 

 tion is itself perpendicular to the axis, the vibrations therefore take 

 place in directions perpendicular to the plane of polarization. 



Trichroitic crystals of course will yield a similar demonstration, 

 as cordierite, andalusite, diaspore, axinite, and others. 



I shall not fail to send a copy of the communication I am to pre- 

 sent today to the Vienna Academy, as soon as it shall have been 

 printed. 



The importance of the subject will, I am confident, plead as an 

 apology for my trespassing on your kindness in thus making the 

 request, that you will lay the present communication before the 

 Royal Society. 



I have the honour to be, 

 My dear Sir, 



Your obedient Servant, 



W. Haidinger. 



2. A Letter to Sir John W. Lubbock, Bart., F.R.S. &c, " On 

 the Stability of the Earth's Axis of Rotation." By Henry Hennessy, 

 Esq., M.R.I. A. &c. Communicated by Sir John Lubbock. 



The author refers to a communication to the Geological Society 

 by Sir John Lubbock, in M'hich he appeals, in support of the possi- 

 bility of a change in the earth's axis, to the influence of two disturb- 

 ing causes, which appear to have almost entirely escaped the notice 

 of Laplace and Poisson in their investigations on the stability of the 

 earth's axis of rotation : — 1 . The necessaiy displacement of the earth's 

 interior strata arising from chemical and physical actions during the 

 process of solidification. 2. The friction of the resisting medium 

 in which the earth is supposed to move. 



With reference to the first of these disturbing causes, the author 

 states, that in his Researches in Terrestrial Physics (Philosophical 

 Transactions, 1851, Part 2.), he has been led to conclusions which 

 may assist in clearing up the question. From an inquiry into the 

 process of the earth's solidification which appears to him most in 

 accordance with mechanical and physical laws, he has deduced re- 

 sults respecting the earth's structure which throw some light on 

 the changes which may take place in the relation between its prin- 

 cipal moments of inertia, which relation is capable of being expressed 

 by means of a function which depends on the arrangement of the 

 earth's interior strata. 



He then states that he has found strong confirmation of his pe- 

 culiar views respecting the theory of the earth's figure, in the expe- 



