The Dispersion and Absorption of Fuchsin 

 which reduces to 



/ (l//t^ — sin'^z — cos /) 



The introduction of the mica at right angles to the ray will 

 shorten its path by (/u, — i)t. The number of wave-lengths retar- 

 dation produced by a piece of mica in the path of the ray when 

 the mica is moved from a position at right angles to the ray. 

 through an angle i, is the difference of these two quantities, and 

 we have at once 



fj\z=i(^\/ fj? — sm^i — cos i) — (fx — i)/ 



In using a compensator with this type of interferometer, there 

 is a shift of the bands due to the lateral displacement of the beam 

 of light, so it is usual to use a double compensator, one piece in 

 each path, thus overcoming the difficulty by moving both pieces 

 together and giving an equal lateral displacement to each beam. 

 The compensator that was used could be turned through 40°, pro- 

 ducing a retardation of three or four bands, before any effect 

 due to this lateral displacement could be observed, and as the 

 greatest shift that had to be measured was less than a band, it 

 was decided to use a single compensator for the sake of con- 

 venience. 



Using the above formula, the values of / that were observed 

 for one, two, and three bands displacement were found to be 

 consistent. Assuming /i to be constant and equal to 1.58 the 

 value of t was found from the formula, and then by substituting 

 back this value the various values of tjX, corresponding to different 

 values of /, were found and a curve of n\ and i plotted. From 

 this the fraction of a band displacement corresponding to any 

 value of i for any part of the spectrum could be determined. 

 There would be a slight error due to the variation of the index 

 of refraction of the mica for light of different wave-lengths, but 

 as this was less than i per cent from each end of the spectrum 

 to the other, the compensator could be calibrated for one part of 

 the spectrum, and a linear correction made in any other part. 

 As a matter of fact, it was calibrated in the red end of the spec- 

 trum, and as the index of refraction of fuchsin is nearly unity 



100 



