478 Prof. Silvanus P. Thompson's Notes 



in principal planes of section, for then, using a cement of 

 given index, the dihedral angle may be made wider (and the 

 prism correspondingly shorter) without risk of having the 

 centre of the field spoiled by total reflection of the extra- 

 ordinary ray. 



To sum up, the advantages of the new prism are : — 



(1) Decrease in length. 



(2) Increase in angular aperture. 



(3) Saving of light consequent on non-obliquity of end- 



faces. 



(4) Minimum of distortion. 



(5) Requires less spar than Hartnack, Glan, or Thompson 



prism of same section. 



Against this are the slight disadvantages — 



(1) Line of section across end face. 



(2) Uses more spar than Mcol of equal section. 



Mr. Ahrens has recently added a thin covering-glass at the 

 end- face crossed by the line $ S', thereby making this line 

 almost imperceptible, as well as affording protection against 

 scratches. Mr. Ahrens has also succeeded in finding a new 

 method of cutting the prism in which there is extremely little 

 waste of spar. 



XL On a Simple Modification of the Nicol Prism, giving 

 wider angle of field. 



As remarked above, in the ordinary Nicol prism the avail- 

 able polarized field is limited, on the one side by the intrusion 

 of the ordinary ray, on the other by the vanishing of the 

 extraordinary ray by total reflection. Of the various methods 

 suggested from time to time for widening the available 

 angular aperture, some have affected one side of the field, 

 some the other, some both. For example, the suggestion 

 made by the writer in 1881 (and by Mr. Grlazebrook in 1882) 

 to alter the prism in such a way as to make the balsam-film 

 a principal plane of section, has the effect (irrespective of the 

 external shape of the prism, which we may suppose given) of 

 throwing back to its furthest possible limit (for any given 

 cement) the point at which the extraordinary ray vanishes by 

 total reflection. The obliquity of the end face (other things 

 being given) affects the limit of intrusion of the ordinary ray 

 to a much greater degree than it affects the extraordinary 

 ray : hence by increasing the slope of the end-faces we may 

 add to the available width of field, but this involves increased 

 distortion of the field as well as loss of light. The use of a 

 more highly refringent cement than Canada balsam causes a 



