390 H. 8. TJhler — Deviation Produced by Prisms, 



absolute index of the material of the prism. The relative index 

 of the prism will be denoted hjn. For the sake of uniform- 

 ity, the definitions of the quantities involved will be the same 

 as given in Southall's "Geometrical Optics." Also, with few 

 exceptions, the notation of this author will be followed. 



In order to Hx the ideas and to make the definitions of the 

 symbols used in the following pages as clear as possible, great - 



Fig. 1 . 



circle and other arcs were drawn to scale on a hemispherical 

 blackboard, the points of intersection were lettered, and the 

 whole was then photographed. In figure 1, which is a repro- 

 duction of this photograph, let the center of the sphere be 

 denoted by O. The great-circle N"^ corresponds to a prin- 

 cipal plane of the prism. One pole of this circle is shown 

 at H. The normals to the incidence and emergence faces are 

 represented by ON 1 and ON 2 , in the order named. With 'N 1 

 and N 2 as centers let arcs of small-circles, AC and BO, be 

 described having such spherical radii as to subtend at the cen- 

 ter of the sphere angles equal to the critical angle, c = sin" 1 — . 



[Only half of each arc is shown in the figure. The points 

 homologous to C and H will be denoted by C' and IF.] These 

 small-circles must intersect if transmission is to be possible. 

 The extremity (S) of the ray inside the prism must not lie out- 

 side of the smaller region bounded by the two small-circle arcs, 

 since total reflection is to be excluded. 



