Sensitive-strip Spectropolariscope. 163 



polariscope of Lippich, in which the field is either bisected or 

 trisected, we have the most adaptable and sensitive instrument 

 which has been devised. This form embodies the very 

 essential feature of a variable sensibility, and, with a sufficiently 

 broad source, a nearly vanishing line is possible. However, 

 in most forms of nicols, since it is the extraordinary ray 

 which is used, there is a displacement of its path. This of 

 course can be avoided if the ray enters normally a nicol with 

 the optic axis in the intersection of its face with its diagonal 

 plane. Most nicols are also found to be slightly prismatic. 

 The ray will usually receive a further displacement on its 

 passage through the second nicol which covers a part of the field 

 of the first one. The two ray systems thus give separate images, 

 which, in spectral work, is a serious difficulty in obtaining a 

 perfect match over the field. In setting for any one colour 

 a rotation of either polarizer or analyser should not displace 

 the image. To avoid this, care must be exercised to eliminate 

 the errors due to displacement mentioned above. Jt does 

 not seem possible to produce a vanishing line between the 

 different parts of the field except with a source of sufficient 

 breadth. With such a radiant at a focus conjugate to the 

 image at the analyser, when the lens is just before the 

 polarizing nicol, there is a great number of bundles of rays 

 which thus produce uniform illumination over the entire field. 

 When, however, the source is very narrow and we have 

 approximately a single bundle of rays, the case is different and 

 we generally observe a decided dark or bright line between 

 the fields, which greatly reduces the delicacy of perception 

 of the eye. This is evidently an irremedial effect since the 

 polished side of the second nicol, usually several centimetres 

 long, will reflect internally or externally any rays striking it, 

 and it is thus impossible to obtain a continuous system of rays 

 across this bounding face, particularly if the rays within it 

 are displaced as described above. In several of the most 

 sensitive half-shade combinations obtainable this defect 

 existed to a greater or less extent. In spectral work, except 

 with bright-line radiants or with absorption cells which never 

 give sufficient homogeneity, a narrow source or slit is essential. 

 Hence the Lippich form of instrument does not seem to be 

 available for general colour observations over successive 

 portions of the spectrum. The amount of light obtainable 

 from a bright-line radiant, e.g. an Aron's lamp, was found to 

 be far below that obtained from the direct rays of the same 

 colour from the sun after passing through a spectral system as i> 

 usually desirable, e.q. absorbing media which have anomalous 



M2 



