382 F. E. Wright — Measurement of Extinction Angles. 



sodium light be used, a plate showing the sensitive violet inter- 

 ference tint is worthless since for that tint the path difference 

 is 572 /x/x, nearly a whole wavelength of Na light (589/a/a), and 

 for this difference K = and fig. 7 applies. If sodium light 

 be used, then plates should be chosen for which the phase 



difference of the two emerging waves is— — — , bright yel- 

 low of the first order or pure yellow of the second order or 

 green yellow of the third order, etc. This is an important 

 consideration and applies to all methods involving the inten- 

 sity equations. 



The visible spectrum includes wave lengths ranging from 

 about 4'00/a/a, and 700/u/x, the interference color in white light 

 ranges from about red of the first order to blue of the second 

 order ; in short, the sensitive interference tint region of the 

 Newton color scale as determined by G. Quincke.* For this 

 interval the distance betw T een the emergent waves is not far 

 from a whole w r ave length for the major part of the visible 

 spectrum ; in other words, the phase difference is such that K 

 is a small fraction not greatly different from zero and the 

 intensity curves for practically all wave lengths w T ill be cov- 

 ered by 5-7. These are, however, the least favorable for 

 showing difference in intensity and such plates are, therefore, 

 the least suitable for the measurement of extinction angles by 

 methods based on intensity differences. On the other hand, 

 plates showing interference colors gray to yellow of the first 

 order are best suited for such measurements. If the methods 

 involving interference tints be used, however, these objections 

 do not hold w T ith equal force. Experience has shown that in 

 case the mineral plate does show red or blue interference tints 

 of the first and second orders the best determinations can be 

 made either by the method of revolving the upper nicol or by 

 the bi-quartz wedge plate, and the true position fixed by noting 

 the absence of abnormal interference colors on revolving the 

 nicol very slightly or inserting the wedge. 



After this digression on the most suitable sections for the 

 measurement of extinction angles, fig. 3 may again be con- 

 sidered and the relative accuracy of the different methods 

 under the same conditions of experiment deduced. 



The heav} 7 curve indicates that for the assumed limit of 

 sensitiveness # 05 per cent of the total intensity, an error of at 

 least ±38' on a single determination is possible by revolving 

 the crystal plate alone under crossed nicols. On the other 

 hand, if the crystal plate remains stationary and the upper 

 nicol alone is revolved, the other intensity curves of fig. 3 are 

 valid, each curve indicating the intensity of illumination of 

 *Pogg. Ann,, cxxix, 177, 1866. 



