the Black Spot in Liquid Films. 515 



ness observed was 13'8 //-//,, while for the most freely exposed 

 the minimum was 7*5 /ifi. 



Effect of an Atmosphere of Carbon Dioxide. — The effect of 

 introducing an atmosphere of carbon dioxide while the black 

 films were formed was remarkable. The films all suddenly 

 became coloured, and did not again thin to the black film. A 

 thick white precipitate formed on the solution at the bottom of 

 the films and gradually covered the surface. A similar pre- 

 cipitate was often noticed at the bottom of the films, even when 

 the case had been sealed, thus indicating that possibly the 

 atmosphere of the films always contained carbon dioxide. 



Conclusions drawn from Results of the First Method. 



Considering the observations here given and many others 

 that were made, these conclusions are : — 



(1) That the thickness of the black film is independent of 

 the proportions of oleate and water. The truth of this may 

 be seen in comparing the readings in Table !._, taken with a 

 1:40 solution, with those of Tables II. and IV., taken with a 

 1: 70 solution. The same is borne out by many other obser- 

 vations on pure oleate solutions varying in proportion from 

 1:40 to 1:100. 



(2) That the thickness of the black film with a sealed atmo- 

 sphere diminishes as the temperature rises. This is illustrated 

 in the curve of fig. 4 (p. 508) most clearly. But it is also 

 shown to be true for sudden changes in temperature in the 

 warming process of all the tables. 



(3) That the thickness of the exposed black film is ultimately 

 that of the second black, which is constant and independent of 

 the temperature. This is shown by comparison of observations 

 I., II., III., and IX. of Table IV. A number of observations 

 indicated that the thickness of the second black film was not 

 constant. However, as the greatest difference in the deflexion 

 corresponding to a certain number of films was never more 

 than a tenth of a fringe, it is difficult to say whether this 

 corresponds to a real difference in the thickness of the second 

 black film or not. 



Method II. 



The idea of determining the thickness of the black film by 

 measuring the intensity of the light reflected at different 

 angles of incidence was suggested by Prof. Michelson. The 

 measurements were made by comparing the images of two 

 slits at the focus of the telescope of a spectrometer. The 

 light from the slit of the collimator entered the telescope after 



