Handbook of Natural Philosopny; Optics (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1866 Excerpt: ... tourmaline, it will be found that the brightness of the surface will vary according to the direction of the axis of the tourmaline, the positions of the axis which render its brilliancy greatest and least being at right angles to each other. That the polarisation in this case is imperfect is demonstrated by the fact that the tourmaline in no position produces a complete extinction of the light. Since light is more or less polarised by successive refractions and by successive reflections, whether regular or irregular, it follows that light is almost never found without being more or less polarised. Thus the light of day proceeding from the solar rays, reflected and refracted by the atmosphere and the clouds, must always be more or less polarised, --an effect which may be p verified by examining this light by one or other of the tests of polarisation, but more especially by the tourmaline already described. 276. Interference of polarised pencils.--If two pencils of light have their planes of polarisation parallel, they will exhibit the same phenomena of interference as have been already described' for ordinary light. The production of bright and dark fringes, when the pencils are homogeneous, and the production of coloured fringes, when the pencils consist of compound light, will occur as in the case of unpolarised light. But if the two pencils be polarised in planes at right angles to each other, none of the phenomena of interference will be exhibited. No matter under what circumstances the rays shall intersect, it can never happen that either ray will extinguish the other, or that the phenomena of dark and light or coloured fringes are produced. When two pencils are polarised in planes forming with each other an oblique angle, they will produce fringes, but...