A treatise on optics / by David Brewster.

  • Brewster, David, 1781-1868
Date:
1831 [i.e. 1840?]
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on optics / by David Brewster. Source: Wellcome Collection.

    257/400 (page 245)
    Fig. 125. m B the prism has every where the same thickness^ it is Fi. 124. obvious that the doubly refracting force varies in different parts of the crystal; but this variation takes place in such a sym- metrical manner in relation to the sides and ends of the prism^ as to set at de- fiance all the recognised laws of crystal- lography. AT^ith the view of observing the form of the lines of equal colouig I immersed the crystal in oil^ and transmitted the polarised light in a direction parallel to a diagonal of the prism ; the effect now exhibited is shown in fig. 125.^ where ABCD is the crystal; AC, and BD, its edges, where the thickness is nothing, and m n the edge through which the diagonal of the prism passes. Now, it is obvious, that if this had been aregular crystal, the lines of equal tint or of equal double refraction would have been all straight lines parallel A C or B D; but in the apophyllite they pre- sent the most singular irregularities, all of which are, however, symme- trically related to certain fixed points within the crystal. In the middle of the crystal, half way between m and n, there are only^i^e fringes or orders of colours ; at points equi- distant from this there are sioc fringes, the sixth returning into itself in the form of an oval. At other two equidistant points near m and w, the 3d, 4th, and 5th fringes D and 7th fringes return into them.selves in the form of a