the Black Spot in Liquid Films, 503 



lasses, to the ends of a zinc trough holding 500 cub. centim. 

 of the solution. This was covered with a glass casing, C 

 (fig. 2), made from a tube by removing a longitudinal section. 

 Two optically plane parallel pieces of glass covered the ends 

 of the case ; while two plane parallels of the same thickness 



Fig-. 1. 



E 



A 



T 



Z 



' I 



*■ 



mimiiMHini 'iPljj ' — ' ' '— '"J s 



were placed at the ends of a similar piece of tubing in the 

 other path of the interferometer. Cementing with beeswax 

 or paraffin prevented any unsteadiness of the fringes, and also 

 provided a practically air-tight chamber for the films. 



Fig. 2 shows a full-sized cross-sectional view of the film 

 box. The frames are shown anchored in the solder at D, 

 while the film is shown formed at H. The temperature 

 readings were taken with a thermometer, T, inside of the 

 case, and held away from the walls by rubber rings. 



Methods of Observation. — By turning the windlasses, the 

 films could be brought into the path of the interferometer and 

 so adjusted that the displaced fringes appeared contiguous 

 with those from the light not going through the films. Using 



2 M2 



