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126                                        ON  SOME  IRIDESCENT  FILMS,                                    [
in air. As in photography it may be rendered insoluble by nitrate of si] acidified with acetic acid, and then exhibits good colours when exarni: under water, with favourable illumination. Filtered gelatine, with wl: a little bichromate has been mixed beforehand, may also be employed, this case the dry film should be well exposed to light before washing. Rea made varnishes also answer well, provided they are capable of withstand the action of water, at least for a time. I have used amber in chlorofo a " crystal" (benzole) varnish such as is, or was, used by photographers, ; bitumen dissolved in benzole. The last is soon disintegrated under wa but the crystal varnish gives very good films. The varnish as sold n probably require dilution in order that the film may be thin enough.
Another varnish which gives interesting results is celluloid in pear-All these films show little in air, but display beautiful colours in water wl the reflexion from the back of the glass is got rid of as already described. 1 advantage from the water depends, of course, upon its mitigating the equality of the reflexion from the two sides of the film by diminishing front reflexion. A similar result may be arrived at by another road if can increase the back reflexion, with the further advantage of enhan< illumination. For this purpose we may use silvering. A glass is coa with a very thin silver film and then with celluloid varnish of suita consistency. Magnificent colours are then seen without the aid of wai and the only difficulty is to hit off the right thickness for the silver. Ot methods of obtaining similar displays are described in Wood's Physi Optics (Macmillan, 1905, p. 142). .bbed at this stage.    If the colours are suitable the plates may now be washed and allowed to dry.    The full development of the colour effects requires that the back of the plates be treated.    In rny experience grinding gives the best results when the lighting is favourable, but an opaque varnish may also be used with good effect.    The comparative failure of such a treatment of the old plates was due to the existence of films upon both sides.    A sufficiently opaque glass, e.g. stained with cobalt or copper, may also be employed.    After the films have stood some time subsequently to the treatment with acid, they may be rubbed vigorously with a cloth even while wet; but one or two, which probably had been rubbed prematurely, showed scratches.