Determination of Refractive Indices II 



If a ray of light, polarized at an angle of 45 ° to the principal 

 axis of a crystalline wedge, pass normally through and is re- 

 flected back again, the emergent light, when resolved by a spec- 

 troscope, will be traversed by interference bands due to double 

 refraction in addition to the ordinary bands arising from the 

 wedge. The superposition of the two sets of these latter bands 

 gives rise to maximum and minimum in the intensity, producing 

 the bands from double refraction referred to. The formula for 



these minima, when the nicols are crossed, is N= \ — . 



A 



In this way both the bands due to interference between the beams 

 reflected between the front and back surface and those due to 

 differential double refraction may be photographed. 



With double refracting crystals, the system of reflection at p x 

 and />„ had to be changed, and is shown in fig. 5. In using the 

 prisms p 1 and p 2 only, there would be a change of phase between 

 the component vibrations. By using a double system of reflec- 

 tion, at right angles to each other, as shown in the cut, the two 

 components will have practically the same change of phase intro- 

 duced. This double reflection will place the image of the slit in 

 a horizontal position, which necessitates turning the wedge so 

 that its length is at rig'ht angles to the image of the slit. 



Plate I is a series of photographs for triple distilled water. 

 The photographs are arranged with the shorter wave-lengths to 

 the left. In each set of photographs No. 4 was the first setting 

 made in the spectrum, and is that of the liquid and air bands 

 together, between the Fraunhofer lines, /? 2 and D v No. 3 was 

 taken in the same portion of the spectrum, after shifting several 

 hundred bands. The others were taken in the order 5, 6, 2, 1. 



In plate I the number of air bands between /?, and D x is 276.6 

 from which the order 1978.5 for the Fraunhofer line D x is ob- 

 tained by the formula N=- -. In the liquid bands there are 



Ai — A2 



374.7 between these two wave-lengths, and after shifting 338 

 bands past the D x line in the spectrum and photographing again, 

 there are by count 326.7 bands in this same region. Substituting 



139 



