398 Transactions of the Society . 



This prism possesses the following advantages : — 



(1) The terminal faces are at right angles to its length ; hence 

 there is very little loss of light through partial reflection of the 

 incident rays, and the lateral displacement of the transmitted beam 

 is scarcely perceptible when the prism is rotated. 



(2) It gives about the same angular field of plane-polarized 

 light as an ordinary Nicol prism, viz. 26°, whUe it is very much 

 shorter, its length being little more than 1^ times its breadth. 

 The actual dimensions of one of the finished prisms are 

 35 mm. x 20 mm. 



One or two points must be attended to in using it. 



(1) It should be so placed that the beam of light may enter 

 the face B C, and not the face in which the edge of the middle prism 

 lies ; otherwise, the emergent beam is mixed with light which has 

 entered on either side of the line of junction, and been partially 

 reflected at the balsam film.* 



(2) The prism must be so placed that the edge A of the middle 

 wedge does not come into focus. This edge can with care be 

 reduced to a line no coarser than a hair, but it cannot be abolished 

 entirely. Hence the prism is not adapted for use as an analyser 

 for the Microscope, the junction-line interfering with the distinct- 

 ness of the image. As a polarizer it answers perfectly if placed in 

 the proper position below the stage, transmitting a broad, clear 

 beam, while it takes up less than half as much space in length as 

 a Nicol prism of the same breadth. As an analyser for the 

 lantern it has been tried by Mr. Lewis Wright and others, and 

 found to answer extremely well. 



* This, however, may be avoided by mounting the prism in a tube which 

 projects about an inch beyond its end, like the hood of a photographic lens ; but 

 this 1b not required, of course, if the prism is placed as above directed. 



