4 5". R. Williams 



secured. The image of the slit at W served two purposes, in 

 that it brought the Fraunhofer lines and interference bands into 

 focus at the same time, and also afforded the means of getting 

 a narrow vertical section of the cell. 



Plate glass cut in strips, 3 x 13 cm. and 6 mm. thick, formed 

 the sides of the wedge-shaped cell. To keep the plates apart, 

 and also form the bottom and ends of the cell as well as the 

 dividing strip between the films, cork strips about .75 mm. thick 

 were cut out, as shown in fig. 2. The plates were then held 

 firmly against this strip by means of clamps, those on one end 

 being screwed down more tightly than the other, in order to 

 make the cell wedge-shaped. The mounting of the cell was so 

 arranged by means of a rack and pinion that it could be raised 

 and lowered in a vertical direction, and also moved in the direc- 



Fig. 2 



tion of its length, by means of a micrometer screw. For photo- 

 graphing the two sets of bands simultaneously, the cell was so 

 moved that the image of the slit occupied a position as ab in 

 fig. 2. 



ABCD is that part of the cell containing the film of reference, 

 while the rest of the cell holds the liquid whose index is to be 

 determined. The two sets of bands having been photographed 

 together, the cell was moved parallel to its length, while the 

 bands from the liquid film were counted as they moved past some 

 fixed point in the spectrum, until the image of the slit occupied 

 a position as cd in fig. 2. In this position a set of bands through- 

 out the spectrum was photographed, and knowing the index for 

 one wave-length, it could be determined for all others by count- 

 ing the bands. 



Air was used for a reference film. Its index has been very 



132 



