14 ■ IV. B. Cartmel. 



is the same as that from glass for wave-lengths longer than 

 640 fxfi. The measurements were, therefore, made with the light 

 comprised between the B and C lines. Sunlight was used and a 

 number of gelatin screens interposed at S, fig. 3, so as to keep out 

 light of those wave-lengths not needed, because the films bleach 

 easily. 



The thickness of the fuchsin may be determined from the two 

 sets of interference bands in several ways, but the method involved 

 in the following formulas, due to Wiener, was the one used: 



We have 



2i—m\,n—{pn^ I )X;„_i.i 



when / is the thickness of the film between the two pieces of 

 glass, \fn the wave-length corresponding to the center of one 

 band, and \ni^\ that of the next band towards the violet. Tak- 

 ing the one of the bands due to the film of air between the glass 

 and the fuchsin, which falls between the 7n^''- and (;«-j-i)^' of the 

 other bands, and calling it A.'»j we have: 



2i'-=1)2)^,n 



where t' is the thickness of the film of air corresponding to this 

 •case. From these, two equations follow at once : 



, A,„ — A ,« A „,_n 



which gives the thickness of the fuchsin t — t' directly. 



The thickness of the air films was regulated so that about five 

 or six interference bands fell between the B and C lines. Thus 

 there were several sets of bands upon which independent mea- 

 surements could be made. 



.Some idea of the accuracy of the measuremenj; can be obtained 

 from the following set of measurements made upon one of the 

 films, in which is included every measurement that was made 

 upon that film : 



114 



