The Dispersion and Absorption of Fuchsin 



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This makes the probable error of a single observation 8/xfiand 

 of the mean 2 fxfx, which is about the order of accuracy that inter- 

 ference methods usually give. 



There is some little uncertainty sometimes as to whether the 

 shift refers to a fraction of a band or to one or two whole bands 

 plus a fraction of a band. In the above instance, the shift is 

 about eight-tenths of a band, and the total displacement one and 

 eight-tenths, giving a thicloiess of about nine-tenths. If the dis- 

 placement had been assumed to be two and eight-tenths bands, the 

 resulting thickness would have been one and four-tenths wave- 

 lengths. To decide this point definitely a number of films were 

 made of different thicknesses. The first of this series was made 

 from a saturated solution, and the rest of the films were 

 made, each succeeding one thinner than the one which pre- 

 ceded it, by continually diluting the solution from which 

 they were deposited. Starting from the thinnest (which 

 was 320 /x/i thick), the thickness of each succeeding one was 

 measured till the thickest was reached (6i7/i.)u,). The continuity 

 of the series furnished data from which the thickness was deter- 

 mined definitely, and observation upon the films with a spectro- 

 photometer confirmed the conclusions. 



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