CONTRIBUTIONS TO SCIENCE. 491 



Optique Physiologique, 1867, pp. 548-554.) All Maxwell's con- 

 clusions regarding Haidinger's Brushes, seem thus to be definitely 

 verified. WILLIAM SWAN, 



Ardchapel, Helensburgh, 2d April 1882. 



2. In the paper read before the Eoyal Society of Edin- 

 burgh on Feb. 18, 1850, Maxwell describes the mathematical 

 results of the application of Stokes's theory of elasticity to a 

 number of cases of the deformation of elastic solids, which 

 results, when possible, he tested experimentally by subject- 

 ing the strained solid to analysis by polarised light. The 

 first, and perhaps the most interesting, example given in this 

 paper, refers " to the case of a hollow cylinder, of which the 

 outer surface is fixed, while the inner surface is made to 

 turn through a small angle." The conclusions derived we 

 give in Maxwell's own words : 



Therefore, if the solid be viewed by polarised light (trans- 

 mitted parallel to the axis), the difference of retardation of the 

 oppositely polarised rays at any point in the solid will be in- 

 versely proportional to the square of the distance from the axis 

 of the cylinder, and the planes of polarisation of these rays will 

 be inclined 45 to the radius at that point. 



The general appearance is, therefore, a system of coloured 

 rings, arranged oppositely to the rings in uniaxal crystals, the 

 tints ascending in the scale as they approach the centre, and the 

 distance between the rings decreasing towards the centre. The 

 whole system is crossed by two dark bands inclined 45 to the 

 plane of primitive polarisation, when the plane of the analysing 

 plate is perpendicular to that of the first polarising plate (see 

 Plate IV.) 



A jelly of isinglass poured, when hot, between two concentric 

 cylinders, forms, when cold, a convenient solid for this experi- 

 ment ; and the diameters of the rings may be varied at pleasure 

 by changing the force of torsion applied to the interior cylinder. 



By continuing the force of torsion while the jelly is allowed 

 to dry, a hard plate of isinglass is obtained, which still acts in 

 the same way on polarised light, even when the force of torsion 

 is removed. 



It seems that this action cannot be accounted for by sup- 

 posing the interior parts kept in a state of constraint by the 

 exterior parts, as in unannealed and heated glass ; for the optical 



