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238 THE ADVANCE OF PHOTOGRAPHY
vantage that there will be very little diffuse deposit, and the resulting colours will appear all the more brilliant, and will also be less affected by the conditions of illumination ; this, however, has the great disadvantage that while it is satisfactory so long as monochromatic light is used, the same is far from being the case when the light consists of complex colours.
The plates are made ready for viewing by cementing on to the film side by means of Canada balsam a thin glass prism, of very small angle, so as to destroy surface reflections, and the back of the glass is covered with asphaltum varnish. Parallel light should be used, if possible, to illuminate the plate, and all side-lights should be cut off — i.e. the parallel beam should strike the plate so that the light may reach the eye at a slight angle from the vertical. If held in the hand and lighted by light coming through a hole in a shutter facing a clear sky, an ideal arrangement would be obtained.
The mercury trough as an adjunct to the dark slide has formed a great drawback to this method, and various attempts have been made to obtain similar results by other means.
Thus Krone relied for his results upon the reflection which takes place at the gelatine-air surface, and dispensed entirely with the mercury. Colours obtained by this method, hoAvever, are very dull and poor. Lehmann flowed the emulsion upon a polished metal plate previously coated with collodion. In this case the polished metal forms the reflecting surface, and the film could be stripped after being exposed, and then floated on to glass.
By this method pure colours can be reproduced, but in those cases in Avhich the lamina? system lies close to the surface of the film near the reflecting surface, failure takes place, since that space is occupied by the collodion. It can readily be seen that a silver mirror in close contact is •open to the same objection. Ives has found that the