Prof. Stefan on the Dispersion of Light by Quarts. 137 



south of its present position. And in this case, the configuration 

 of the land being supposed to have been similar to what it is at 

 present, a greater proportion of the current would be turned into 

 the southern branch along the Brazilian coast, and of course a 

 correspondingly smaller proportion would flow into the Gulf of 

 Mexico. The Gulf-stream would consequently be greatly di- 

 minished, if not altogether stopped. 



If a relation could be properly established between geological 

 epochs and changes of climate, due to the change in the excen- 

 tricity of the earth's orbit, we might then have some hope of being 

 able to arrive, at least approximately, at a knowledge of the posi- 

 tive ages of the various strata composing the earth's crust. The 

 total age of the crust itself, as we have already noticed, can be 

 determined by other means. Taking the temperature of melting 

 rock at 7000° ¥., Prof. William Thomson has calculated, from 

 principles of cooling established by Fourier, the probable age of 

 the earth's crust to be about 98,000,000 years*. The entire 

 geological history of our globe must therefore be comprehended 

 within this period. 



As yet no calculations have been made regarding the time 

 when the excentricity was at a maximum, or of the time required 

 to pass from the maximum to the minimum state of excentricity. 

 In the Annates of the Paris Observatory, vol. ii. p. 29, there 

 is a Table giving '0473 for the excentricity at 100,000 years 

 before the year 1800, and -0189 for the excentricity 100,000 

 years after 1800. There are subordinate maxima and minima 

 in that interval of 200,000 years; but the principal maximum 

 I have been informed does not fall within that period. We 

 may therefore safely conclude that it is considerably more than 

 100,000 years since the glacial epoch. 



XIV. On the Dispersion of Light by Quartz, owing to the Rota- 

 tion of the Plane of Polarization. By Prof. Stefan f . 



THERE are only two possible forms of dispersion ; to each 

 colour in white light may be assigned either a particular 

 velocity of propagation or a particular direction of vibration. The 

 first kind of colour-dispersion occurs in refraction and diffraction ; 

 the second kind when light passes through a substance which 

 turns its plane of polarization, inasmuch as the rotation has a 

 different magnitude for each colour. 



A spectrum resulting from the alteration of the plane of po- 



* Phil. Mag. ? or January 1863. 



t Translated by Prof. Wanklyn from the papers issued by the Kaiser- 

 Hche Akademie der Wissenschaften in Wien, 18G4, No. 15. 



