358 Prof. t). B. Brace on AcJiromatic Polarization 



right angles they were in subtractive positions. The bands 

 thus formed were seen diagonally across the faces of the 

 wedges, the dark band usually used as a test of the order of 

 a plate thus covering different orders in each wedge. The 

 degree of blackness and of decolorization of this band was an 

 indication of the amount of achromatism. 



In order to give a greater range to the observation without 

 cutting a number of wedges of successively higher orders 

 compound wedges were used, see fig. 2. 



Fig. 2. 



Thus a and b were two wedges cut so that when super- 

 posed in the reverse direction they bore the same relation to 

 the principal axes. The angle of a was made slightly different 

 from b so that the system formed a compound wedge of small 

 angle, over which several bands of varying order might be 

 formed by sliding b with respect to a. By using c either in 

 subtractive or in additive order, the lowest orders up to the 

 highest could be examined. On placing a and b, with or 

 without c, over a corresponding system of another crystal, 

 broad bands could be obtained by proper adjustment of orders 

 up to several hundred or a thousand. On placing selenite 

 over quartz a perfectly colourless dark band was obtained for 

 the zero and lowest orders, but this became gradually more 

 and more coloured as the orders increased up to the fiftieth, a 

 decidedly bluish band being obtained at this point, this 

 indicating compensation for the red end of the spectrum, 

 which is in agreement with the observations made by the first 

 method, while the adjacent higher band was bordered by a 

 pink and green and showed scarcely any blue, indicating com- 

 pensation for the blue as found previously. With still higher 

 multiples the coloration was increased. 



Iceland spar and quartz showed a nearly black line with a 

 bluish tint for the orders twenty-two Iceland spar and twenty- 

 three quartz respectively, indicating fairly good achromatism. 

 Multiples of these gave greater coloration, but less than the 

 selenite and quartz for similar orders. 



