1877.] 



Thickness of Soap Films. 



337 



descent of the line of separation between each pair of colours could be 

 determined and its position calculated at the moment when any particular 

 electrical observation was made. 



In this calculation the velocity of the motion of each colour was sup- 

 posed to be uniform between successive observations. This supposition 

 was to a certain extent justified by the fact that when several measure- 

 ments were taken, the calculated velocities for consecutive intervals of 

 time differed but little for the thinner parts of the films. In the thicker 

 parts a greater irregularity was observed, but the exact determination of 

 the position of the boundary between two colours was also of less im- 

 portance. In making these observations a screen was placed behind the 

 film, and the telescope of the cathetometer was directed to a portion of 

 the film illuminated by means of a mirror so placed that the direction of 

 the incident light was perpendicular to the optic axis of the telescope. 

 The film was thus viewed by light incident at 45°, and the refractive 

 index of the liquid being known the thickness corresponding to any 

 colour may be deduced by means of the table of Newton's rings given in 

 Watts's ' Dictionary of Chemistry/ 



The refractive index was found to be 1*395 for mean yellow rays. 

 It was determined, not from the liquid used, which was made a week 

 before the experiments began, but from another freshly made solution of 

 precisely the same composition. This course was rendered necessary 

 by the fact that such solutions rapidly become turbid, owing to the forma- 

 tion of a precipitate which cannot be sufficiently removed to enable the 

 liquid to be used for such a purpose. 



Table I. 





I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX. 



X. 



XI. 



XII. 



Black 



mm. 



286-90 



mm. 

 286-25 



mm. 

 •65 



o-oo 



10-00 



10-00 



•065 



286-25 



345 



401 







Yellow 2 



2404 



1-000 



Orange 2 

 Purple 3 

 Green 3 



282-00 

 280-35 

 279-00 



281-15 

 279-75 

 278-50 



•85 

 •60 

 •50 



0-75 

 3-00 

 3-75 



11-00 



11- 75 



12- 25 



10-25 

 8-75 

 8-50 



•084 

 •069 

 •060 



281-23 

 279-87 

 278-63 



502 

 1-36 

 1-24 



430 

 525 

 630 



2096 

 1733 



1005 

 1-004 



Bed 3 



277-30 



276-75 



•55 



5-25 



13-25 



8-00 



■069 



276-98 



1-65 



825 



1376 



1-008 



Green 4 







Green 4 



276-35 



275-90 



•45 



6-25 



14-00 



7*75 



•058 



276-13 



0-85 



950 



1127 



1002 



Bed 4 



























987 



1003 



Bed 4 



273-40 



268-00 



5-40 



7-25 



14-25 



7-00 



•771 



271-27 



4-86 



1077 







Green 5 







Green 5 



















11-97 



1150 



870 



1-000 



Beading for the top of the liquid cylinder 289 - 7 



bottom „. ...... 259-3 



2 A 2 



