The Philosophical magazine; a journal of theoretical, experimental and applied physics . calcareous crystal becomes separated into tworays polarized in planes at right angles to each other; and inNicoPs rhomb one of these falls so obliquely upon a commonsurface of the crystal and a cement of Canada balsam as to betotally reflected and thrown aside, whilst the other traverses thefilm of (Jement, passes through the spar beyond it, and finallyemerges from the rhomb as a ray polarized in one plane. Abeam of light passing through the rhomb in this manner,becoming plane-polarized and of great bright
The Philosophical magazine; a journal of theoretical, experimental and applied physics . calcareous crystal becomes separated into tworays polarized in planes at right angles to each other; and inNicoPs rhomb one of these falls so obliquely upon a commonsurface of the crystal and a cement of Canada balsam as to betotally reflected and thrown aside, whilst the other traverses thefilm of (Jement, passes through the spar beyond it, and finallyemerges from the rhomb as a ray polarized in one plane. Abeam of light passing through the rhomb in this manner,becoming plane-polarized and of great brightness without colour,the rhomb is of continual use in optical experiments. J\I. Fou-cault has, it appears, adopted this explanation, and brought itbefore the British Association at the late Meeting in Dublin, aswell as the form of rhomb which I described in the above-namedtreatise, with air in place of Canada balsam between the compo-nent prisms. On examining a XicoPs rhomb of the original form, it will befound to be like fig. 1, where two like prisms of calc-spar areFip;. 1. FiK. cemented together with Canada balsam in the common surface* Commiuiictitcd by the Author. Prof. Potter on the Principle of NicoPs Rhomb. 453 adbc; and taking the principal section aghf, the angle a hf willbe found to be about aright angle. Let ag hf, fig. 3, representthe same section as in fig. 1. Let S k be a ray of light enteringthe rhomb at k, and separated by the double refraction into anordinary ray k o, and an extraordinary one ke; and if the ray S kis incident nearly perpendicularly upon the surface bf the ordi-nary ray k o will have very little deviation in direction; but theextraordinary ray, as Huygens found, will have the angle ekoabout 6^ 40; an inclination which is towards the obtuse summitof the rhomb. Now the film of Canada balsam being representedby b a, the angle of incidence of the ordinary ray k a may be sogreat that the ray cannot pass into the Canada balsam, but willbe totally
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