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XLVII. On the Properties of Compound Double-refracting 

 Rhumbs. By Professor Pottee, A.M.^ 



IN the Philosophical Magazine for December 1857, I entered 

 into some discussions of the properties of Nicol's rhomb, 

 and also of other forms of rhombs, for procuring a beam of 

 plane-polarized light. I there stated that I believed the expla- 

 nation of the manner in which Nicol's rhomb furnishes a beam 

 of plaue-polarized light by one of the double-refracted rays un- 

 dergoing total reflexion, had not been published before it ap- 

 peared in my experimental treatise on Physical Optics. I have 

 found, since writing that paper, that the credit of first giving 

 that explanation belongs to M. Radicke, who gave it in his 

 Handbuch der Optik, vol. ii. p. 369 ; and he has a discussion 

 of the subject also in the Annalen der Physik und Chemie for 

 1840. 



With respect to the computations given in my paper of De- 

 cember 1857, those for Nicol's rhomb are upon the only method 

 we can follow until we know the specific refraction of the two 

 double-refracted rays on passing from calc-spar into Canada bal- 

 sam, and they are sufficient for the approximate solution. The 

 method followed in that paper for the case when air is left be- 

 tween the component prisms is, however, so defective for the ex- 

 traordinary ray, that the results must be rejected as erroneous. 

 A strict numerical computation might be obtained without a 

 very great deal of trouble if the subject were worth it. 



I have now to describe the practical results afforded by the 

 rhomb of calc-spar and glass described in the paper, which is 

 similar in form to the common double-image rhomb, but with 

 air between the component prisms in place of Canada balsam, 

 and especially the particular form described in the last sentence 

 of the paper. In that form a rhomb of calc-spar is cut in a 

 plane through the optic axis, and this is made one of the outer 

 faces of the compound rhomb, with a polished natural cleavage 

 plane for the inner face next that of the glass, as at a 6 of fig. 4 

 of the paper. When wc place a rhomb thus constructed before 

 the eye with the calc-spar nearest to it, and look directly towards 

 the daylight, we sec two boundaries of total reflexion, one for the 

 ordinary, and the other for the extraordinary rays, with the in- 

 terval of S*' or 9° between them, which is of plane-polarized 

 light ; and this is the analogous beam to that which is employed 

 when Nicol's rhomb is used, where a plane-polarized beam of 

 27^ is obtained. When the glass portion is nearest the eye, only 

 one boundary of total reflexion is seen ; but the two beams 

 having an angular separation of about 11°, another boundary of 

 * Communicated by the Author. 

 2E2 



