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236 THE ADVANCE OF PHOTOGRAPHY
high as possible, it is best to use a developer like pyrogallic acid. This gives a deposit which, while being fairly transparent, is yet highly reflecting ; on the other hand, where there is a complex arrangement of colours to be reproduced, it is best to work with a deep-acting developer, such as hydroquinone, and then bleach so as to obtain a transparent deposit.
The production of good whites has always been a difficult matter by this process, and has been the subject of much discussion. According to Lippmann's theory white is produced as the result of light being reflected from particles of silver which are densely but irregularly distributed throughout the film, so that in such a region regular-spaced laminae would be entirely wanting. Cajal, on the other hand, came to the conclusion that white can be produced only by the use of amidol as developer, and the subsequent use of an intensifier, since, in his opinion, it is due to a mirror-like surface being formed on the film. Others, however, have been able to obtain very satisfactory whites without the use of amidol.
There is sometimes, especially with under-exposed plates, a greenish tint to be seen in the whites. Lehmann considered this to be due to the formation of a rapidly damped stationary vibration of a wave-length equivalent to the mean wave-length of the incident light. He endeavoured to correct this by using a screen having three maxima of transmission, viz., red, green and blue, since on short exposures white will be produced as the result of a mixture of these three colours ; but there are serious objections to such a practice.
Ives finds that the brightest whites are obtained by using an emulsion containing four times the amount of silver used for pure colour work, or twice that quantity which has been commonly used by other investigators, a fact which tells very strongly in favour of the theory of scattered reflecting particles. He also finds that Isocol