174 



Sir J. Conroy. On the Polarisation of [Feb. 4, 



of double refracting substances, and showed that Malus' statement 

 with regard to Iceland spar was incorrect. 



Malus said that Iceland spar behaves towards the light it reflects 

 like a common transparent body, and that its polarising angle is about 

 56° 30', and that whatever be the angle comprehended between the 

 plane of incidence and the principal section of the crystal, the ray 

 reflected by the first surface is always polarised in the same manner 

 (" Theorie de la Double Refraction," pp. 240, 241). 



Some years later Seebeck (" Pogg. Ann.," vol. xxi, p. 290 ; vol. xxii, 

 p. 196 ; vol. xxxviii, p. 276 ; vol. xl, p. 462) made a number of very 

 accurate observations on the same subject, and in 1835 and 1837 

 Neumann published in " Pogg. Ann.," vol. xl, 497, and vol. xlii, p. 1, 

 an account of further experiments that he had made on the reflection 

 of light by Iceland spar. 



He begins his second paper by a brief summary of the results 

 obtained by Brewster and Seebeck. " Brewster found that the angle 

 of complete polarisation for calcspar depends on the position of the 

 reflecting surface relatively to the axis, and upon the position of its 

 principal section to the plane of reflection ; he also found that 

 when the reflecting surface is covered with a liquid, the plane of 

 polarisation of the completely polarised ray does not coincide with the 

 plane of reflection, but makes a smaller, or greater, angle with this ; 

 when a cleavage-face of calcspar is covered with oil of cassia this 

 deviation may amount to 90°. The knowledge of these phenomena 

 has only been further advanced in recent times. Dr. Seebeck has so 

 followed out, by means of most accurate determinations, the influence 

 of optically uniaxial crystals upon complete polarisation, that the 

 angle of incidence at which this occurs can be determined as accu- 

 rately beforehand as it can by Brewster's law in the case of uncrystal- 

 lised bodies. Seebeck also discovered that the deviation of the plane 

 of polarisation from the plane of reflection, which Brewster had 

 observed, also occurs when the ray of light falls directly from air on 

 to the surface of the crystal." 



Seebeck's observations having been mainly directed to the deter- 

 mination of the angle of polarisation, Neumann's object was to deter- 

 mine the azimuth of the plane of polarisation of the reflected light. 

 They both assumed, contrary to Fresnel's hypothesis, that the density 

 of the ether in the two media was the same and the elasticity different, 

 and therefore that the plane of vibration coincided with the plane of 

 polarisation, and starting with this assumption succeeded in showing 

 that the observed and calculated results were in close accordance. 



Seebeck and Neumann only repeated a portion of Brewster's experi- 

 ments, and no one except Sir David Brewster appears to have made 

 any determinations of the angles and azimuths of polarisation when 

 the spar was in contact with media other than air. 



