the Colours of Mixed Plates. 689 



In the formula, i and r are the angles of incidence and 

 refraction at the surface of the meniscus, and i and r' are 

 the angles of incidence and refraction at the rear surface 

 of the plate. 6\ as before, is the phase-change at total 

 reflexion. 



In fig. 2 (c) we have one ray undergoing two refractions 

 at the surface of the meniscus, and the other ray passing 

 wholly through the liquid and totally reflected at the 

 meniscus. The path-difference in this case is 



/*/ (cost — sin r — i) —t cos r — 8 



and sin 6 = \x sin (2r— i — yjr). 



Fiff. 8. 



1 (a) * 



hi fig. 3 (a), (/>), (c) we have the corresponding cases ot 

 the meniscus concave to the incident ravs. 



