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LII. On the Principle of Nicol’s Rhomb, and on some improved 
forms of Rhombs for procuring Beams of plane-polarized Light. 
By Professor Porrer, 4.M.* 
Nee advantages of the compound rhomb of calc-spar, dis- 
covered by the late Mr. Nicol, in many of the interesting 
experiments of physical optics, are familiar to most who have 
tried experiments on the polarization of light. The theory of 
the mode of action of this rhomb by one of the doubly refracted 
rays undergoing total reflexion, had not however been published, 
that I am aware of, before it appeared in my experimental trea~ 
tise on Physical Optics, published in the autumn of last year, 
although I had given that explanation to my friends for many 
years past. The explanation is as follows:—A ray of light en- 
tering a face of calcareous crystal becomes separated into two 
rays polarized in planes at right angles to each other; and in 
Nicol’s rhomb one of these falls so obliquely upon a common 
surface of the crystal and a cement of Canada balsam as to be 
totally reflected and thrown aside, whilst the other traverses the 
film of cement, passes through the spar beyond it, and finally 
emerges from the rhomb as a ray polarized in one plane. A 
beam 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. M. Fou- 
cault has, it appears, adopted this explanation, and brought it 
before the British Association at the late Meeting in Dublin, as 
well as the form of rhomb which I described in the above-named 
treatise, with air in place of Canada balsam between the compo- 
nent prismns. 
On examining a Nicol’s rhomb of the original form, it will be 
found to be like fig. 1, where two like prisms of cale-spar are 
Fig. 2. 
cemented together with Canada balsam in the common surface 
* Communicated by the Author, 
