472 Mr. H. P. War an : Interferometer Method of determining 



usual brass mount, but provided with a trough arrangement 

 as shown in fig. 1 in part section, by which clean mercury 

 could be introduced below the plate and made to cover its 

 bottom surface. This combination being of identical size with 

 the usual mount of the parallel plate could be fitted on the 

 Hilger interference spectroscope between the collimator and 

 the constant deviation prism, as in fig. a (PL VIII.). 



Fig. 1. 



The slit was illuminated by a mercury vapour lamp, and the 

 resulting interference pattern displayed an interesting change 

 as shown by fig. b (PL VIII.). For the first appearance it 

 was as if an extra system of bands had crept in between the 

 normal system of bands. That it was not due to a simple 

 reflected system was evident from the absence of constant 

 spacing between the two sets of bands. An idea that it 

 might be due to a thin layer of air between the glass and the 

 mercury led to a repetition of the experiment by silvering 

 the bottom surface of the plate chemically. The same type 

 of band system observed made it conclusive that it was being- 

 brought about as an effect of the metallic reflexion at the 

 bottom surface. 



Considering the effect of such a metallic reflexion, we 

 know from the optical properties of metals that the emergent 

 light will be elliptically polarized. On this consideration, 

 the extra system of bands could be explained away directly 

 as due to a simple phase difference effect brought about by 

 the metallic reflexion. In such a case the effect of polarizing 

 the incident light plight to be to make one of these two 

 systems of bands disappear as shown in fig. 2, according as 

 the incident light is polarized in the plane of incidence or 

 perpendicular to it. A Nicol polarizer introduced into the 

 path of the incident light brings about this change as 

 expected, confirming the validity of our explanation. 



