658 



Prof. J. Patterson on the Electrical 



That is, the number of band-widths X n is displaced towards 



2 



which 



the red end of the spectrum, gives a multiple of 

 is the thickness required. 



The displacement of one set of bands relatively to the 

 other was measured with the spectrometer. The optical glass 

 was placed in front of the slit of the collimator (tig. 6). 



Fig. 6. 



The line of junction of the two laminae was placed so as to be 

 perpendicular to the slit. 



The light from a Welsbach entered the collimator through 

 an opening A, and part of it was reflected by the glass mirror 

 C through the collimator-slit, and fell at perpendicular inci- 

 dence on the air-film. It was then reflected back through 

 the collimator and glass prism D and viewed by the telescope 

 E. Two sets of dark bands were thus obtained in the spec- 

 trum, one of which was displaced relatively to the other. By 

 measuring the angular width of each band and the displace- 

 ment of the corresponding band in the other set, the thickness 

 of the metallic film was obtained from formula 1. The 

 spectrometer was calibrated by means of known lines and a 

 curve was drawn connecting wave-lengths and the angles of 



Fig. 7. 



the graduated circle. From this curve the wave-length corre- 

 sponding to any angle could be obtained. 



The chief difficulty in measuring the thickness arose from 

 the indefiniteness of the dark bands. By depositing a very 

 thin film of metal on the cover-glass so that the interference- 



