Twin Prisms for Polarimeters. 397 



is greater the less the radius of the sphere. Hence, at a 

 given time since consolidation, the rate of rock-folding on 

 the smaller of two planets is to that on the larger in a greater 

 ratio than the surface of the former to the surface of the 

 latter. 



6. Nothing can be inferred from this as to the relative 

 heights of individual mountain-ranges on different planets. 

 If, however, at the same periods of their history, the 

 mountains on a small planet be of the same or less average 

 height than on a large one, then the rate of continental evolu- 

 tion on the former must, area for area, be greater than on the 

 latter ; or, cceteris paribus, the continents of a small planet 

 encroach upon its ocean-areas more rapidly than do those of a 

 larger one. 



Perhaps this may in part account for the possibly advanced 

 state of development of the planet Mars, as indicated by what 

 seem to be its comparatively extensive continents and narrow 

 ocean-beds. 



LIL Twin Prisms for Polarimeters. 

 By Professor Silvanus P. Thompson, D.Sc* 



TO explain the points of novelty in the new twin-prisms 

 now exhibited by the author, a brief resume of some of 

 the recent advances in polarimetry is needed. In the earliest 

 apparatus, dating from the time of Biot, the polarizer was 

 usually a bundle of glass plates, the analyzer a simple double- 

 image prism or Nicol prism provided with a divided circle to 

 measure its rotation. With the subsequent substitution of 

 compensators and of spectroscopic apparatus in the analyzing- 

 portion of the apparatus, this paper has no concern, inasmuch 

 as the prisms to be described are intended to serve as polar- 

 izers only, not as analyzers. 



When the polarizer was a mere bundle of plates, or a Nicol 

 prism, producing simple plane-polarized light of approximately 

 homogeneous complaneity all over the visual field of the 

 apparatus, exact measurements of the angle of rotation were 

 not easy, simply because the eye failed, through a certain 

 range of angle, to determine the precise position of the 

 analyzer giving maximum extinction of light. For more 

 exact polarimetry, Soleilf, in 1845, introduced the biquartz 



* Communicated by the author, having been read, before Section A 

 of the British Association Meeting at Manchester. 



t Comjrtes Rendus, xx. p. 1805, 1845 ; xxi. p. 426, 1845 ; xxiv. p. 973, 

 1847 ; and xxvi. p. 163, 1848. 



