Differential Double Refraction. 541 



having their principal axes parallel to those of the corre- 

 sponding larger wedges and the same angle, and C, C plane 

 plates used to obtain higher orders than are possible with the 

 wedges alone. L is a platform, moveable by means of the 



Fk. 1. 



screw D, on which 



totally-reflecting prism, F the collimating-lens, and G a flint- 

 glass prism placed at the angle of minimum deviation for 

 ]Sa light. H I is the observing-telescope, and K a photo- 

 graphic scale placed at the principal focal length from the 

 achromatic lens J. 



The collimator-slits S, S were ruled on silvered glass with 

 a sharp knife. These slits should be close to the wedges, 

 through which the light passes normal to their optic axes. 



