STUDY OF ORES AND METALS. 



433 



angles 'to that of the axis in the second. The relations are shown in 

 Fig. 8. If the combination be observed between crossed nicols with 

 the axes at 45° with the principal nicol planes, a series of parallel, 

 vertical dark interference bands appears, if monochromatic light be 

 used, in the field ; if a white light source be used the bands, except 

 one central band, are colored. These bands are similar in appearance 

 to the bands in a quartz wedge or a Babinet compensator ; but their 

 mode of formation is different as they depend on very slightly con- 

 vergent polarized light for their development while in the quartz 

 wedge the change in thickness of the wedge introduces the required 



Fig. 8. Diagrams illustrating the principle on which the construction of 

 the Savart plate is based. 



path difiference. From the mode of formation it is evident that if 

 non-polarized light be used no such bands will appear, but that with 

 a small percentage of polarized light there will be superimposed on 

 the white field a series of colored bands the intensity of which in- 

 creases with the amount of polarized light present. Under the best 

 conditions of setting at the center of the light bands the maximum 

 intensity is obtained, namely, J/^ of total non-polarized light and Yi 

 of total polarized light ; whereas at the center of the dark central 

 band only the non-polarized portion is transmitted. At intermediate 

 points the non-polarized light and a part of the polarized component 

 is transmitted. The field, therefore, alternates in intensity ; the least 

 amount of polarized light which can be detected depends obviously 

 on the least perceptible increment in intensity which the eye is able to 

 detect. As noted above Koenig and Brodhun's data place this dif- 

 ference limen at 1.6 to 2 percent, for favorable intensity of illumi- 

 nation. Pickering and others^ estimate that in sky polarization 

 about I per cent, of polarized light can be detected. Koenigsberger 



^ Report of U. S. Naval Observatory of Total Eclipse of July 29, 1878. 



PROC. AMER. PHIL. SOC, VOL. LVIII, BB, JAN. 21, I92O. 



