To obviate this, glass in different colours was tried, and long rectangular wedges with regularly graded tapers were ground and polished for standards, whilst correspondingly tapered glass vessels were made for the beers. These were arranged to work side by side, and perpendicularly, before two apertures of an optical instrument, which gave a simultaneous view of both. The apertures were provided with a fixed centre line, to facilitate the reading off of comparisons of thickness. There was no difficulty in obtaining glass which approximated to the required colour when used in one thickness only. But as thickness varied, the test no longer held good for both standards, their rates of colour change being different, making the method unreliable.[1] [1] It was afterwards found that these colour changes through variations of intensities were due to a natural law to be described under the heading of “Specific colour.” (_See_ page 32.) The system about to be described is one of analytical absorption, and has been published from time to time in the form of papers, read before Societies interested in the question of colour standardization; as also in two descriptive works by the present writer. The earlier works were necessarily fragmentary, but gathered system as the subject progressed. At an early stage in the investigations it was realized that the handbooks of the period dealt largely with theoretical differences which were of little service to the technical worker. Under these circumstances the writer applied for advice to the late Mr. Browning of the Strand, who gave it as his opinion that no work existed which could be of service to the writer. All that could be done was to go on until something should be arrived at. On this, all theoretical reading was put aside, and the work proceeded on the simple lines of observing, recording, and classifying experimental facts. In working with glass of different colours it was found that some combinations developed colour, whilst other combinations destroyed it. This suggested the probability of a governing natural law; and experimental work was undertaken in the hope of discovering it. The result was the construction of a mechanical scale of colour standards, which are now in use in over one thousand laboratories, and no question of their practical accuracy arises. The principal conditions for ensuring accuracy and constancy of results are embodied in the following code of nine precautions, which have been published for nearly twenty years without being disputed. They may therefore be considered as governing laws, at least for the present. The colour theory adopted for these Governing Laws has grown out of a series of experimental facts capable of demonstration, and is summed up in the following code of nine Laws. Laws 1, 2, and 3 relate to White and Coloured Light, and are as follows:--