MEASURING DRY MASSES WITH INTERFERENCE MICROSCOPES 223 



In the first instance, the plate L is assumed to be transparent and 

 the whole arrangement placed on the stage of a polarizing interference 

 microscope. The differential method is used. The liquid drop may 

 also be put on the reflecting plate L. Phenomena are then observed 

 by reflection although the principle of the method is unaltered. In the 

 second instance (Fig. 8.15) the liquid is enclosed in a micro-chamber, 

 set on the microscope stage, the plate L being immersed vertically. 



FiG. 8.15. Solid plate L immersed in a liquid. 



Let us consider, for instance. Fig. 8.14. The angle Q is the tangent 

 angle with the horizontal line A' M at the surface of the liquid. The 

 thickness of the liquid drop, in the area through which the ray (1) 

 passes is e. The thickness e,„ is maximum in the area (2). n being 

 the index of the liquid, the optical path between A and A' is Jq 

 = (/z— 1)^ + £»„,. Assuming the angle d to be small, then, according 

 to equation (7.16) 



(«-l)0. (8.6) 



a 



The angle a is the slope of the transmitted surface wave H (Fig. 8.16). 

 According to equation (7.13), then 



d = 



a 



{n-\)d 



(8.7) 



The tint at P is determined, which yields b, and a is derived by 

 determining the tint outside the drop. The contact angle 0^. sought 

 is derived by measuring b quite close to Af. When a solid plate is 

 immersed (Fig. 8.15) the contact angle is y and then, y ^njl—d. 



Let us now assume ihat, in the flat areas outside the drop (Fig. 8.14), 

 the instrument is so adjusted as to show the first order purple (0-565 fx). 

 If the second order sky blue is observed at M (0 664/0. then b^a 

 = 0099 //. To compute d in formula (8.7) the data required are the 

 duplication <:/, characteristic of the interference microscope and the 



