The yield in the dye vat may not be in direct relation to the solutions in water, the establishment of this is a question for the expert, and presents no apparent difficulty.
The use of the scheme in recording the degree of fading of dyes has been previously dealt with in the Journal (q.v., 1908, p. 36).
Limitations and Precautions.—It has been shown that we have analytical control, within certain limits, of light and colour under daylight conditions.
The general limits for colourless light range from total darkness to 28 units, when the unabsorbable red ray comes into evidence.
For colour, the general limits range from 28 to 18 units, between 18 and 15 all colours become indistinct, but at varying rates, below 15, colour is not distinguishable.
The principal disturbing conditions in making observations are want of colour education and insufficient diffusion. In the case of the latter, the first evidence is the disturbance of constancy by the penetrating red ray. A partial remedy is to interpose a white diffusion screen, such as tissue paper.
Time of Observation.—This should not exceed five consecutive seconds, as the keenness of perception decreases by time, but varies for different colours.
Angle of Incidence.—Sixty degrees is safe for most solids, but for bright or polished surfaces, such as varnishes, polished metals, etc., the angle must be lessened as the degree of smoothness increases. For very rough surfaces, such as loosely woven stuffs, etc., care must be taken that the lay of the fibre is uniform.
Distance from the Object.—Ten inches has been adopted for general work, but certain visions require more or less as their focus varies from normal.