The only two positions between which choice usually lies are therefore those in front of (b) or behind (d) the lens of the camera.
In the circumstances usually met with in practice, a filter placed in front of the lens does not alter the focus,' which is a very appreciable advantage in the case of cameras with which the focussing is done on a graduated scale. On all other cameras this position of the filter lends itself most readily to taking on and off with the minimum of trouble. The filter can either be mounted in a ring, which is fitted over the lens like a cap, or provided with threads which allow of it being screwed on to the lens hood. In short, in all cases where the object to be photographed is more than twice the focal length distant from the objective, this position is, in the case of thick filters, that which causes the least risk of introducing disturbing aberra tions into the image.' 122. Care of Light-filters. When not in use, all light-filters should be protected from the action of light, since the dyes with which they are made are not invariably unaffected by its prolonged action, and thus changes in their absorptive power may be caused in time.
When cleaning filters cemented between plate glass, the same precautions should be observed as when cleaning photographic lenses. Further, water should never be allowed to come into contact with the edges, since any wetting of the gelatine film would cause the latter to swell and might cause deformation or even separation of the glasses.
Plain gelatine should never be handled except by their edges or between fine tissue paper any contact with the fingers invariably leaves fingermarks which cannot be removed and which seriously impair the definition. These screens must be protected from heat and damp, and when not in use should be kept between the leaves of a small notebook of white paper. When it is desired to cut out a circle of gelatine filter for fitting into a lens, breakage of the film can best be avoided by cutting it out between two pieces of strong paper, one of which has the circle to be cut out marked on it. As gelatine
screens continually undergo slight expansions and contractions according as the air is more or less damp, they should never be fitted into any kind of rigid frame. Lastly, gelatine filters should never be kept between plate glass unless cemented on both sides with Canada balsam (solution in xylene). In this way multiplicity of reflecting surfaces and risks of tearing the gelatine will be avoided.
122a. Polarizers. In a normal condition, light vibrations, perpendicular to the direction of propagation (§ 2), are scattered at random in all possible directions. Various conditions can polarize light, that is, preserve only the vibra tions parallel to a given plane, called plane of polarization. For instance, the light diffused by a blue sky is polarized, and this all the more completely as the sky is clearer. The light re flected on a non-metallic surface is polarized, the polarization being maximum (without, how ever, being complete) when the reflected rays are perpendicular to the refracted ones (reflec tion under an angle of 35° with the reflecting surface in the cases of glasses with a refractive index of 1-5). If the surface is transparent the refracted light is also polarized, and it is then possible to decrease the proportion of non polarized light by causing the light to pass through several thin plates under the same orientation. The light that has passed under a given incidence a suitable assembly of double refracting colourless crystals (Nicol or Glaze brook prisms, made of Iceland spar) is totally polarized. Light that has passed a double reflecting crystalline dichroic plate (tourmaline, Heraphathite or iodosulphate of quinine) or a film holding in suspension a multitude of double-refracting, dichroic, ultramicroscopic crystalline needles similarly orientated (luteo cobaltic perioclosulphate ; E. H. Land, 1934) is, at least in a spectral interval comprising the great majority of visible rays, formed mainly of polarized light.