400 Twin Prisms for Polarimeters. 



by the author in 1881, have several advantages over ordinary 

 Nicol prisms. Their polarized field is more homogeneous, 

 their transverse section is rectangular, their terminal faces 

 are at right angles to the axis of vision ; and by the method 

 of construction adopted it is easy to obtain from one piece of 

 spar two identical prisms cut from a single rectangular block. 

 These points are all of advantage for the present purpose, 

 namely, that of perfecting the apparatus for use in precise 

 polarimetry. 



Prisms cut on Ahrens's plan also possess another pecu- 

 liarity ; the terminal faces are, as mentioned above, principal 

 planes of section, but the crystallographic axis lies in these 

 faces at almost exactly 45° with the rectangular edges. 

 Hence it comes about that, if one of these simple prisms be 

 reversed end for end, while used as either polarizer or analyzer, 

 the dark field is changed to bright field, or vice versa, the same 

 as if the prism had been rotated about its axis of vision 

 through 90°. This is, of course, not the case with any of the 

 ordinary Nicol, Foucault, or Hartnack prisms. Suppose, 

 then, that two identical prisms, cut as described, are taken, 

 of narrow rectangular cross-section, having the sides of the 

 section in the proportion of 2 : 1. Let them be placed side 

 by side so that, as the light falls through them, the visible 

 juxtaposed fields of vision form the two halves of a square. 

 Let one prism be then reversed end for end. The juxtaposed 

 field of vision would still be a square divided into two halves ; 

 but these two halves will be oppositely polarized. Such a 

 twin-prism can be used for polarimetry exactly as the Picker- 

 ing prism or as these half-shadow combinations, in which there 

 is a fixed angle of 90° between the planes of polarization of 

 the two halves of the visual field. The first twin-prism now 

 exhibited by the author is of this nature. 



In the second form of twin-prism, the same mode of con- 

 struction is followed so far as the cutting of two identical 

 prisms of narrow rectangular section is concerned ; but, after 

 the prisms have been cut, a small wedge of about 2° 30' is 

 ground away from the side of one of them, and they are then 

 again juxtaposed, so that there is this amount of angular dis- 

 placement between the two halves of their joint visual field. 

 In both cases the two prisms are ground together and 

 cemented by balsam, so that the observer sees merely a fine 

 line of junction down the middle of the visual field. 



The author considers such constructions superior to that of 

 Righi, because there is much less loss of light by reflexion at 

 oblique surfaces ; he also prefers his method to the methods 

 of Laurent and of Poynting, because there is no difference in 



