and Differential DdWle Refraction. 355 



orders seven quartz and eight mica gave also very good 

 coincidence, better than nine quartz and ten mica, which 

 might indicate a ratio between the first two ratios but nearer 

 the first, namely, eight quartz and nine mica. For these 

 orders the quartz bands were slightly closer than those of the 

 mica in the extreme blue, but somewhat more extended in 

 the red. On increasing the thickness of each and noting the 

 passages of the sodium band a second coincidence was ob- 

 tained for the orders fifteen quartz and seventeen mica, and 

 also for sixteen quartz and eighteen mica. The next approxi- 

 mate coincidence was obtained with twenty-three quartz and 

 twenty-six mica, and also a closer one with twenty-four quartz 

 and twenty-seven mica, the latter giving more complete co- 

 incidence for the central band. The orders eight quartz and 

 nine mica were estimated to give the best ratio for achro- 

 matism, although each may be slightly too great. The co- 

 incidence of the bands was not as perfect, however, as with 

 the selenite and the quartz for the same orders, the deviation 

 of the quartz being at least one-tenth of a band within that 

 of the mica in the blue and extending two-tenths in the red 

 beyond those of the mica. 



Comparison of Mica and Selenite. 



The wedges and plates of mica and selenite used previously 

 in the comparisons with quartz were compared directly. 

 With four bands in the field a first coincidence was obtained, 

 the order for each crystal being found to be eight for mica 

 and seven for selenite. The selenite bands in the blue were only 

 perceptibly shorter and also longer in the red. On increasing 

 the orders of each successively a second coincidence was 

 obtained for the sixteenth order of the mica and the fourteenth 

 of the selenite. A closer coincidence was obtained here than 

 for the second coincidence of the mica and quartz. A third 

 coincidence was obtained for the twenty-fourth order of mica 

 and the twenty-first order of selenite. Only slight discre- 

 pancies could be observed, the coincidence being almost 

 perfect in the yellow and green. The ratio of eight mica to 

 seven selenite being undoubtedly very approximately the best 

 achromatism. 



Comparison of Iceland Spar and Left-lianded Quartz. 



Comparisons were made with these two crystals, but dif- 

 ficulty was experienced in obtaining a wedge of the former 

 of sufficiently low order and good definition to make accurate 



202 



