2SS Prof. Anderson and Mr. Bowen on an 



observed. Hence it was concluded that the angles of contact 

 in the cases examined were zero. 



As the liquid on the two sides does not evaporate at quite 

 the same rate, the upper edges on the two sides do not always 

 coincide : thus a better examination can be made if only one 

 side of the plate be wet and only half the drop shown in the 

 figure be formed. 



If the argument used above is correct, it is possible 

 to use the experiment as a rough method of measuring 

 surface-tensions. To do this we consider the convex lens 

 also, which has its centre at 0', and which brings the parallel 

 light to a focus at B. 



Let a = OA. 

 b = O'B. 



]i = vertical distance between OA and O'B. 

 r Y — radius of curvature of each face of concave lens 



(assuming curvature the same). 

 r 2 = radius of curvature of face of convex lens. 

 fjb = refractive index of liquid. 

 p = density of liquid. 

 p 1 = pressure inside liquid at 0. 



P2= ,5 _ >3 J3 O'. 



ir = atmospheric pressure. 

 T = surface-tension of liquid. 



Treating the lens as thin, 



1 2 1 2 



i=(^~l)-, £=G*-1)-, 

 a yn ' t x b J r 2 



T 

 T 



••• »-*= T fe + ^ric^(;r + -i)- 



But P2—Pi = 9ph '■> 



.'. T = 2gp(fi-l)habl(a + b). 



If only one side of the plate is wet with the liquid, ithe 

 formula becomes 



T=gp(fi-l)hab/(a + b). 



