on some New Polarizing Prisms, 479 



gain on the side of the extraordinary ray — it thrusts the blue 

 iris further back — but causes a slight loss on the side of the 

 ordinary ray, which intrudes a little more than before. 



Now, taking the Nicol prism as it is ordinarily made, there 

 would be a real gain, if it could, without additional labour or 

 cost, be so cut as either to widen the field (using the same 

 length of prism as before) or to shorten the length of the 

 prism (if obtaining the same angular aperture as before). 

 The method of cutting adopted by Hartnack, and that sug- 

 gested in 1881 by the present writer, both add to the cost of 

 spar and to the labour of cutting. In Hartnack's construc- 

 tion the width of field is gained partly by employing linseed 

 oil, partly by the device of making the plane of the film lie 

 at right angles to the crystallographic axis of the spar. In 

 the writer's own construction of 1881 the balsam-film was 

 made to lie in a principal plane of section, whilst the prin- 

 cipal axis of vision through the prism was made to lie at 

 right angles to the crystallographic axis. A gain of about 

 9° in the width of the field over that of a Nicol of the same 

 external form was the result ; being a little less for flat- 

 ended prisms, a little more for oblique-ended prisms. 



The author has now devised a simple modification of the 

 mode of cutting the Nicol prism, which possesses several of 

 the advantages of these costlier methods of construction, but 

 without adding appreciably to the cost. 



Fig. 5 shows the ordinary Nicol prism as usually cut, the 

 end-faces A B and C D being natural faces of the crystal 

 polished up. The books assert that makers of Nicol prisms 

 cut down these faces, making them still more oblique by 3°. 

 I have not found any constructor who does this, Mr. Ahrens 

 does not ; Messrs. Steeg and Reuter do not ; M. Lutz does 

 not. The natural angle between the face A B and the arrete 

 A D is about 109°. The crystallographic axis makes about 

 45° with the end-face A B. The balsam-section is at about 

 90° (it is exactly 90° in all Mr. Ahren's prisms ; it is a little 

 less in some of Dr. Steeg's) to the plane of the end-face. 

 The consequence of this is that there are about 45° between 

 the plane of the balsam-film and the crystallographic axis. 

 This limits the field : those rays which traverse the prism at 

 small angles to the film, and which would traverse the film 

 if the crystallographic axis were at right angles to it (as in the 

 Hartnack prism) are totally reflected out, because the crystal- 

 lographic axis slopes at 45°. 



To remedy this I cut the crystal in the manner shown in 

 fig. 6 or fig. 7. Fig. 6 represents a piece of spar of the same 

 size as fig. 5. The end-faces are first cut away about 40° 



