IV 



Proceedings. \October 5th, i8gy. 



index, AC being unity and constant, B being movable as desired. 

 At D is a slide guiding two equal cords of length EA, secured at 

 E, C and B. The rods AB and AC terminate in pointers on a 

 protractor F. 



By moving the slide D towards or away from A the varying 

 angles of incidence and refraction are shown by the pointers. 



Another construction which lends itself to instrumental 

 requirements more perfectly, however, is shown in Fig. 4, in 

 which BG is the mean proportional between BD and BC. 



Fig. J,. 

 In my latest instrument, of which Fig. 5 is a diagrammatic 

 representation, the above construction is utilised. 



A is a slide capable of being moved along the diameter of 



Fig. 5. 



