1919-20.] An Unnoticed Point in Theory of Newton’s Rings. 55 
Brewster took air, soap solution, and diamond for his three media, 
giving respectively 
r Y = 1 for air. 
?’ 2 = 1*475 for soap solution. 
r 3 = 2*44 for diamond. 
In this case we have 
Also 
and 
Hence 
^3 = 2-44 
^= 2 * 176 . 
t*i >*3 — r 2 2 — *164. 
This condition approximates closely to the case in which r 1 r 2 = r 2 2 , and 
explains why Brewster obtained a bright spot at the centre and variations 
in intensity approximating to darkness when cos (9 — 0, since case (a) is 
avoided and case ( b ) (ii) is highly exceptional. 
( Issued separately March 25, 1920.) 
