Intelligence and Miscellaneous Articles. 69 
ON A NEW FORM OF POLARIZING PRISM, BY C. D. AHRENS. 
The prism which I desire to bring to the notice of the Society is 
intended for use either as a polarizer or an analyzer. It will, I 
hope, be found especially useful as an analyzer for the microscope. 
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 off. Double-image prisms of the 
usual construction are shorter, but they have another 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. 1). Fig. 1. 
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 lies in a 
plane bisecting the refracting angle. This 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 Rochon’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 
may be practically disregarded altogether; an eye, even when 
placed almost close to the prism, receiving only the direct beam. 
This beam is, of course, perfectly polarized in one plane, and can, 
by a proper arrangement of the glass compensator, be rendered 
practically free from distortion and colour. 
Other methods of effecting the compensation have suggested 
themselves in the course of my work; and I have obtained the 
best results by adopting the arrangement represented in fig. 2. 
