Sensitive-strip Spectropolariscope. 167 



strip as compared with the long polished side of the second 

 nicol in the Lippich form, a much more continuous system of 

 rays is possible and hence a far sharper bounding or a 

 vanishing line between the fields. 



Various systems have been tried for obtaining different 

 monochromatic light of sufficient intensity. As mentioned 

 above, the intense system of Aron gives only definite wave- 

 lengths and hence cannot be used for all purposes. Any one 

 of the mercury lines was also found to be much less intense 

 than a corresponding portion of the spectrum sufficiently nar- 

 row to give, within the limits of observation, the same rotation 

 of all its components in the experiments tried. Absorption 

 cells are not sufficiently monochromatic for many observations 

 and are of course restricted in the colours available. Except 

 in the sensitive-strip system here described, a broad diffusing 

 source is necessary. Here too sunlight gives the best results; 

 but in order to use it as such, a diffusing plate, e.g. a ground- 

 glass or milk-glass plate, must be inserted. This is found to 

 reduce the available light by 1)5 per cent, to 98 per cent. Owing 

 to the sensibility of the system any arbitrary spectral arrange- 

 ment will not suffice. No light should be allowed to enter 

 the polarizing system which is not used in the immediate 

 observations. The spectral system adopted must fulfil these 

 conditions. The most obvious arrangement and one used 

 largely is to place a slit at a focus conjugate to the analyser 

 and with the lens immediately at the polarizer, a dispersing 

 prism being placed immediately before or after the lens. A 

 spectrum is thus formed in the plane of the analyser and by 

 a proper diaphragm or slit in this plane, the eye, aided by a 

 telescope or not, will see the field illuminated with a uniform 

 tint. This is owing to the fact that the retina is a conjugate 

 focus to the field aperture, which is a colour radiant, and its 

 Jens converges all rays of different colours radiating from one 

 point of the field (dispersing prism) to a common focus on 

 the retina. If, however, a cell of absorbing substance, say for 

 the green, is placed in the path of this beam, the very small 

 amount of green which is transmitted may be largely 

 neutralized by extraneous light or by the smallest fraction of 

 red and blue which is diffused from the faces of the prisms 

 .or from internal reflexions of the system, which thus becomes 

 a radiant. This has been found to give apparent rotations 

 following some regular law predicted on entirely different 

 grounds. To avoid these spurious results, the entire 

 dispersion and separation of the colour should take place 

 outside and beyond the polarizer. This of course may be 

 attained in many ways, several of which have been fcried, 



