[Illustration: Fig. 177.--A Rhomb showing the passage of Rays of Light.] Mr. Nicol, of Edinburgh first succeeded in making a rhomb of Iceland spar into a _single-image prism_. His method of splitting up the crystal into two equal parts was as follows:-- A rhomb of Iceland spar of one-fourth of an inch in length, and about four-eighths of an inch in breadth and thickness, is divided into two equal portions in a plane, passing through the acute lateral angle, and nearly touching the obtuse side angle. The sectional plane of each of these halves must be carefully polished, and the two portions cemented firmly together with Canada balsam, so as to form a rhomb similar to that before division; by this management the ordinary and extraordinary rays are so separated that only one is transmitted: the cause of this great divergence of the rays is considered to be owing to the action of the Canada balsam, the refractive index of which (1·549) is that between the ordinary (1·6543) and the extraordinary (1·4833) refraction of calcareous spar, and which will change the direction of both rays in an opposite manner before they enter the posterior half of the combination. The direction of rays passing through such a prism is indicated by the arrow, Fig. 178. [Illustration: Fig. 178.] Polarised light cannot be distinguished from common light, as already said, by the naked eye; and for all experimental purposes in polarisation, two pieces of apparatus must be employed, one to produce polarisation, and the other to show or an analyse it. The former is called the _polariser_, the latter the _analyser_; and every apparatus that serves for one of these purposes will also serve for the other. [Illustration: Fig. 179.--Polariser.] [Illustration: Fig. 179_a_.--Analyser.] Polarising Apparatus for Students’ Microscope. In all cases there are two positions, differing by 180°, which give a minimum of light, and the two positions intermediate between these give a maximum of light. The extent of the changes thus observed is a measure of the completeness of the polarisation of light. The two prisms mounted as shown in Figs. 179 and 179_a_ constitute the apparatus adapted to the microscope. The polariser slips into place below the stage, and the analyser, with the prism fixed in a tube, is screwed in above the objective. The definition is considered by some experimenters as somewhat better if the analyser be used above the eye-piece, and is certainly more easily rotated. [Illustration: Fig. 180.--Prism mounted as an Eye-piece.]