. Polarisation of light . e pair of discs to be opened and shut like apair of pincers. If a crystal plate be inserted betweenthe two, and the whole held close to the eye, the raysfrom parts of the field at different distances from thecentre will reach the eye, having traversed the crystalwith different degrees of obliquity ; and a system ofrings and brushes will be formed. VIII.] POLARISATION OF LIGHT. 99 Another method consists in applying to Norrem-bergs polariscope a pair of lenses, one below thecrystal with the crystal in the focus, the other aboveit. The first ensures that the rays shall
. Polarisation of light . e pair of discs to be opened and shut like apair of pincers. If a crystal plate be inserted betweenthe two, and the whole held close to the eye, the raysfrom parts of the field at different distances from thecentre will reach the eye, having traversed the crystalwith different degrees of obliquity ; and a system ofrings and brushes will be formed. VIII.] POLARISATION OF LIGHT. 99 Another method consists in applying to Norrem-bergs polariscope a pair of lenses, one below thecrystal with the crystal in the focus, the other aboveit. The first ensures that the rays shall traverse thecrystal with different degrees of obliquity ; the secondbrings within the range of vision rays which wouldotherwise fail to reach the eye, and at the same timeconverging them into a cone with a smaller verticalrange, renders the ring smaller than when seen withthe simple tourmalins. An additional lens of greaterfocal length, of less power, is often added in orderto adjust the whole to individual Fig. 22. Fig. 22 gives the general appearance of the ad-dition to the apparatus of Norremberg describedabove, and Fig. 23 the course of a system of raysbrought to a focus on the crystal by the lens a b, andagain converged by a second lens c d. But by far the most successful arrangement for H2 I CO POLA RISA TION OF LIGHT. [chap. viil] polarisation of light, ioi enlarging the field of view so as to comprise thecomplete system of rings even with bi-axal crystalshaving widely inclined optic axes, is the system oflenses due in the first instance to Norremberg. Thedisposition of the parts is shown in Fig. 24 ; and thegeneral appearance of the instrument as constructedby Hofmann of Paris, and called by him the Polari-microscope, is also given, Fig. 25. In this instru-ment the lenses which converge the rays upon thecrystal plate can be taken out, and replaced byothers giving parallel light; it can then be used asan ordinary polariscope. Mention has been made above o
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