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184 THE ADVANCE OF PHOTOGRAPHY
and each person had some particular wrinkle of his own in this direction. Necessarily, therefore, very indefinite ideas were sometimes obtained from accounts given of this or that developer, as certain alterations were prescribed for correcting faults in the exposure, etc., for it was believed that by careful developing, incorrect exposure could be very largely rectified.
Constant Density Ratio. — It was Hurter and Driffield who were responsible for the earlier work upon the laws which govern the densities of the photographic plates, or, in other words, the quantity of silver which can be deposited under certain conditions by the action of the developer. Their researches formed the turning-point in the method of dealing with this question.
The}7 came to the conclusion, that the relationship existing between the densities and the light intensities is determined by the exposure, and that it cannot be altered by modifying the constitution of the developer or by the time occupied by developing the plate. This is often spoken of as the law of " Constant density ratios," and it can be imagined that such a law was not at first considered as of very great importance by the practical photographers of the time. Their opposition was very largely based upon wrong ideas, for ammonia and a soluble bromide being then commonly used in developers (their proportions being arranged to suit the plate), the density ratios obtained were quite irregular and could not be controlled.
To understand the meaning of " Constant density ratio," let us imagine that a strip of plate has been exposed for a certain definite time — say one second — to a given source of light, that a second strip has been exposed for two seconds to the same source of light, and that both are developed for five minutes. On measuring the densities of the strips at the end of this time suppose that the second is 16 times as dense as the first.
Now expose similarly two other strips and carry on