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XV. — On a New Form of Polarizing Prism. 

 By C. D. Ahrens. 



{Read 11th June, 1884.) 



The prism which I desire to bring to the notice of the Society is 

 intended for use either as a polarizer or an analyser. It will, I 

 hope, be found especially useful as an analyser for the ]\Iicroscope. 



The employment of a Nicol prism above the eye-lens is 

 subject to the great inconvenience that, owing to the necessary 

 length of the prism, the eye of an observer is so far removed 

 from the lens that a portion of the field is cut ofi". Double-image 

 prisms of the usual construction are shorter, but they have ano,ther 

 defect, viz. that the angular separation of the rays is so slight 

 that the eye sees both images at once, and some confusion is thus 

 caused. 



My object in constructing this improved prism has been to 

 obtain a much wider separation of the two beams of light ; so 

 that one of them, although not actually removed entirely by total 

 reflection (as in the Nicol prism), is so far refracted to one side 

 that it may be neglected altogether. I made several attempts to 

 construct such a prism some years ago, but failed (as probably 

 others have done) owing to the difficulty or impossibility of avoid- 

 ing distortion and colour, and of obtaining a wide separation of the 

 ordinary and extraordinary rays in a prism made up of only two 

 pieces of Iceland spar. 



I have now effected the desired object by making the prism of 

 three wedges of spar cemented together by Canada 

 balsam, as shown in the accompanying drawing (fig. 87). Fig. 87. 

 The optic axis in the two outer wedges is parallel to 

 the refracting edge, while in the middle wedge it is 

 perpendicular to the refracting edge, and hes in a 

 plane bisecting the refracting angle. 'J'his disposition 

 of the optic axis is the one originally suggested by 

 Dr. Wollaston, and has the effect of causing a greater 

 angular separation of the rays than Kochon's construc- 

 tion. By the employment of three prisms instead of 

 two I am able to give the middle prism a very large 

 angle, and yet to correct the deviation of the rays so far 

 that on emergence they make approximately equal angles with the 

 central line of the combination. 



Nearly in contact with one of the terminal faces of the prism 

 I place a prism of dense glass of such an angle that it just corrects 

 the deviation of one of the rays and also achromatizes it, while it 

 increases the deviation of the other ray to such an extent that it 



