Heinrich Wilhelm Dove

“ It is preferable to graduate that side of both plates which is turned toward the eye. The dotted stand in fig. 2. is therefore to be imagined behind the plate, when it belongs to the polarizing prism, and on the contrary- before the plate and the graduation on the back of the plate, when the plate belongs to the analysing prism . It will seldom be requisite to alter the plane of polarization of the incident light; it is most convenient to place it once for all horizontally, that is to say, to place the index of the polarizing prism upon 0° or 180°. ”

“ If this explanation is correct, precisely the same phænomena would be obtained by gradual heating as by successive reflexions in the interior of the Fresnel's rhomboid, but with this difference, that instead of the direction of the polarization varying by successive steps we should expect a continual transition through all degrees of elliptic polarization. The experiments confirm this perfectly. ”

“ As is a constant quantity for a definite substance, the difference will be proportional to the quantity . Hence it follows, That when for one definite colour the light is circularly polarized by an interposed crystallized lamina, it may for the other colours be linearly and oppositely circularly polarized, and that the difference between the single colours increases with the thickness of the lamina and with the intensity of the double refraction. ”

“ The lower end of the glass plate heated upon the hot steel plate cools when the lamp is taken away quicker than the upper one, to which heat is also communicated by con- duction. After some time therefore the centre of the plate becomes its warmest part. As the lower end, standing upon the rapidly cooled conductor of heat, becomes still cooler, the warmer spot moves upwards until finally the upper angle becomes the warmest. ”

“ Since now the crystal in this ring may also revolve in its plane, its optical axes may be altered at will in reference to their position with respect to the plane of polarization of the incident light. ”

“ The thicker the plate the nearer to each other are the differently-acting vacant spaces. ↑ Brewster says in reference to the colours which fluor spar acquires by rapid cooling, "Fluor spar was very slightly affected." ”

“ The graduation of the circle is arranged so that, in the vertical position of the stand, the straight line passing through the points 0° and 180° lies horizontally. Fig. 2. exhibits of the actual size a view of these plates, which are not drawn in fig. 1. ”