6 W. B. Cartmcl 



The bands as viewed with the system just described presented 

 the appearance represented diagrammatically in fig. 2, and 

 showed the anomalous dispersion of fuchsin in a general way at 

 a glance. 



FigZ 



Air "bandg. 



Violet 



Ked. 



PuchsiJi bands - 



In discussing these bands we will, for the sake of clearness^ 

 call those bands affected by the fuchsin the fuchsin bands, and 

 the others the air bands. The bands as shown in fig. 2 corre- 

 spond to a film too thin to produce a whole band retardation, so, 

 at the two points where two sets of bands coincide, the index of 

 refraction is unity. To determine the index of refraction in any 

 other part of the spectrum, one of the mirrors is moved parallel 

 to itself by means of a screw till a fuchsin band appears in the 

 required portion of the spectrum, unless indeed one is already 

 there. Now, by means of a compensator, an air band is brought 

 into coincidence with the fuchsin band, and the amount of re- 

 tardation introduced by the compensator is equivalent to the retar- 

 dation produced by the fuchsin. In the case of a film thick 

 enough to produce more than one band displacement, the fraction 

 of a band is measured by means of the compensator, and the 

 whole number of bands added to this. 



It was found that when an attempt was made to produce an 

 arbitrary shift of a definite fraction of a band, either by moving 

 one of the mirrors or rotating a compensator, the bands were 

 disturbed by the mere touching of the mirror or compensator. A 

 very thin mica compensator, thin enough so that it had to be 



106 



