432 



SUMMAKY OF CURKENT RESEARCHES RELATING TO 



Index Limiting 



inaex. Angle. 



Canada balsam 1-549 .. 69° 1' 



Balsam of copaiba .. .. 1-507 .. 65° 3' 



Linseed oil 1-485 .. 63° 6' 



Poppy oil 1463 .. 61° 9' 



For tbe first three substances, even tbe extraordinary ray which 

 grazes the surface A B does not undergo reflection. The plane A B 

 forms the other limit of the field. It is not the same for poppy oil : 

 the extraordinary ray is reflected at an incidence of 79° 9' represented 

 by the dotted line P'. 



In the interior of the spar the extent of the field with these 

 different substances is as follows : 



Canada balsam 20° 9' 



Balsam of copaiba 24° 7' 



Linseed oil 26° 4' 



Poppy oil 17° 0' 



The position of the faces of incidence and emergence must be such 

 that the rays which limit the field on the two sides are equally 

 inclined to these faces so as to give a field symmetrically disposed 

 relatively to the axis of the prism. As on one side the limiting ray 

 is refracted according to the ordinary index, whilst on the other 

 side it is refracted according to the exti-aordinary index, the faces of 

 incidence and emergence cannot be perpendicular to the line which 

 divides in two the field in the interior of the prism. These faces 

 should have a less inclination on the side of the ordinary ray than on 

 the other. 



In the calculation of this inclination there is a consideration 

 which must not be omitted : it is that the dispersive power of spar for 

 the extraordinary ray is higher than that of the fatty oils, and that 

 their relative indices diminish in passing from red to violet. At 

 about the limit of the field the blue and violet rays will still traverse 

 the film of oil, whilst the red are arrested. The field is terminated 

 on this side by a somewhat broad violet band which darkens it to a 

 certain extent. It is, therefore, necessary to arrange to have on this 

 side a larger portion of the field so as to employ only the part which 

 is the most uniformly illuminated. 



In consequence of this the following angles have been adopted, the 

 crystal being supposed to be cut perpendicularly to the principal axis 



Canada . . 



Copaiba . . 



Linseed . . 



Poppy .. 



Length of the 

 prism. 



mm. 



52 



37 



34 



30 



