Sensitive-strip Spectropolariscope. 



169 



C-line. 



D-line. 



E-line. 



•075° 



•095° 



•1575 



•075 



•095 



•1575 



•0775 



•100 



•1600 



•075 



•1025 



•1550 



.075 



•095 



•1550 



•075 



•0975 



•1550 



•070 



•0975 



•1575 



•075 



•0975 



-1575 



•075 



•0975 



•1525 



•075 



•1000 



•1550 



•075 



•095 

 •095 

 •095 

 •095 

 •095 



•1525 



when it is near the system, by housing it in a thick iron 

 cylinder of suitable length which becomes at once the 

 containing tube for the cells. Attention should also be 

 called to the further fact that when the rotation is measured 

 immediately by the analyser the different elements of the 

 field sometimes show a slight difference in tint if the rotation 

 is considerable. This is due to the fact that the light is 

 not strictly homogeneous, and the different components are 

 rotated by different amounts. This could be wholly or 

 partially avoided by using a rotary compensator, e. g, quartz 

 wedges. 



For perfect adaptability to any wave-length there is no 

 doubt that systems giving channeled spectra are preferable, 

 but on this account only, as their sensibility is far less than 

 the half-shade or sensitive-strip instruments. A rotation 

 equal to the amount observable in the system described and 

 referred to in the table above means a displacement of only 

 one forty -thousandth of the distance between two black bands 

 in a channeled spectrum. It is very doubtful indeed whether 

 a setting in channeled spectra closer than one degree can be 

 made with the same certainty as one two-hundredth of a 

 degree with this system. This means one two-hundredth o\' 

 a band, which ordinarily is very high indeed. However, some 

 observers have claimed settings as accurate as from ten to 

 three one-hundredths of a degree or one six-thousandth of a 

 band. Experienced observers in repeating such observations 

 with similar apparatus have never been able to set. with 

 certainty, closer than one or two degrees. Such a realizable 

 sensibility would truly be an immense step in advance, as a 



