ZOOLOGY AND BOTANY, MIOROSCOPYj ETC. 



577 



angles respectively made by the film and by the terminal planes with 

 the axis of the prism to be the same as in the Nicol prism. 



It is possible to produce a further increase in width of available 

 aperture at the other side of the field by reflecting back the ordinary 

 ray more than in the Nicol prism by making the terminal faces 

 more oblique ; but there is then more loss of light by reflection at the 

 surfaces. 



Besides the advantage of a wider angular aperture, this new form 

 of polarizing prism has the advantage of producing a field in which 

 the rectilinear polarization approximates more uniformly and sym- 

 metrically to a polarization in one plane than is the case in the 

 ordinary Nicol. There is, however, more waste in cutting the spar, 

 with proportionate increase in cost. 



Prof. Thompson has been good enough to supplement his original 

 paper by the following additional remarks and diagrams. 



Fig. 108 is the ordinary Nicol prism, as cut from a symmetrical 

 rhomb of spar. (Such a rhomb might be split so as to give four 

 ordinary Nicols.) 



Fig. 109 is the Hartnack prism, so cut that the film lies at right 

 angles to the optic axis. 



Fig. 110 is the Thompson wide-angled prism, cut so that the film 

 of balsam is in a principal plane of section, and the longitudinal axis 

 at right angles to the optic axis of the crystal. 



Fig. 108. 



Fig. 109. 



Film inclined about 

 23° to long axis of prism. 

 End faces mostly make 

 68° with the long axis. 

 Optic axis inclined at 

 about 43° to planes of 

 end faces. (X Y optical 

 axis). 



Film perpendicular 

 to optic axis. Longi- 

 tudinal axis of prism does 

 not lie at right angles to 

 the optic axis of the 

 crystal. 



Film runs from front 

 edge a 6 to the back edge 

 behind c d, and therefore 

 it is in a principal plane 

 of section and contains 

 the optic axis, and the 

 optic axis is at right 

 angles to the longitudinal 

 axis of the prism. 



The reason why the new method of cutting secures a wider angle 

 to the field of the extraordinary ray may be further elucidated by the 

 diagrams figs. Ill and 112. 



Fig. Ill is to demonstrate the point that in the ordinary Nicol, 

 although the maximum refractive index of extraordinary rays is 

 less than the refractive index of Canada balsam, yet that at certain 

 angles of incidence the extraordinary ray does not pass through 

 the balsam film, but suffers total internal reflection. In this figure 



Ser. 2.— Vol. III. 2 P 



