284          ON THK  CHAXUK  OF   UKFRANUIBUJTY  OF  LIGHT.
The arrangement being the same as in the third method, but the lens in front of the prisms  having a focal length of only 6*5 inches, the incident light was polarized  in a vertical plane previously to passing through the slit, by transmission through a pile of plates.    The two beams of light were seen as usual in the fluid, namely, the blue beam due to internal dispersion, and the fainter coloured beam due to motes.    The former of these, which was quite separate from the latter, exhibited the principal fixed lines belonging to the highly refrangible part of the spectrum.    A plate of selenite was then interposed immediately in front of the vessel, so as to modify the polarization of the light entering the fluid.    This plate was obtained by an irregular natural cleavage, and was cemented with  Canada  balsam  between  two discs of glass.   When examined by polarized light it exhibited a succession of beautiful and varied tints, according to the various thicknesses of the different parts.    Now when the plate was moved about in front of the vessel, without altering its perpendicularity to the incident light, different portions of the beam due to motes were observed to disappear and reappear, or at least to become faint and then bright again, so that a person ignorant of the cause, and not looking at the disc, might have supposed  that the observer had been holding in front of the  vessel  a  piece of dirty glass, having the dirt laid on in patches; but in whatever manner the disc was moved in its own plane without rotation, or turned round an axis perpendicular to its plane, not the slightest perceptible change was produced in any part of the blue beam.
21. In all the experiments described in this paper in which a spectrum was formed for the sake of examining the separate action of portions of light of different rcfrangibilities, the length of the spectrum was horizontal, so that the fixed lines were vertical. Nevertheless it will be convenient, for the sake of shortness, to use the prepositions above and below to signify respectively on the more refrangible side of and on the less refrangible side of.
The principal fixed lines of the visible spectrum will be denoted by letters in accordance with Fraunhofer's admirable map. These lines are now too well known to need description.