A Spectrometer for Electromagnetic Radiation 
197 
plates for this experiment. Fig. 5 shows the displacement of the 
first two maxima caused by the insertion of a plate of paraffine 
5, 3 cm. thick. From this curve it appears that the first maximum 
is shifted 2.9 cm. and the second 2.7 or a mean of 2.8 cm. 
Then the refractive index ^ = I,ir6 
5-3 ^ 
Another similar experiment gave ^ 
mean 1.52 
(8) Diffraction Phenomena. A variety of diffraction effects can 
be shown with such an apparatus, such as the spreading of the 
radiation after passing through a narrow aperture, the reflection 
of a larger amount of energy to a given receiver by a small plane 
mirror than by a considerably larger one diffraction bands by 
the use of a slit opening having a width suitably related to the 
wave-length employed, etc. Fig. 6 shows the first two ‘Tright 
bands” with the intervening ‘Mark band” obtained by the use of a 
slit, 17.6 cm. wide, interposed between exciter and receiver. In 
such experiments the receiver must be used without its usual 
