The measurements were made at two-inch intervals, and the four perpendicular lines are at the colour points selected for valuing the four distinguishing marks, technically known as “Water White,” “Superfine,” “Prime,” and “Standard.” Intermediate qualities find their position in the scale of curves according to their measured colour values. This method of standardizing commercial values has also been adopted by the International Tanners’ Association, the Inter-States Cotton Seed Oil Association, and other oil industries. Also for scale, solid fats, and such substances as can be easily melted and measured by transmitted light. =Varying Effects of Different Lights.= _Pathological Applications._--The law of specific colour development was made use of by Dr. George Oliver in determining the degrees of hæmoglobin in the blood. The method is fully explained in his Croonian Lecture before the Royal College of Physicians of London, July 11, 1896. _Detection of Forgeries._--The system and apparatus is used by Professor A. S. Osborne, Examiner of Questioned Documents, New York City, for determining the variety of ink, the age of the writing, and the detection of forgeries. A full description of the process will be found in his work entitled _Questioned Documents_, published by the Lawyers’ Co-operative Society, Rochester, N.Y. [Illustration: PLATE IX SIX ANILINE DYES. CURVES ILLUSTRATING THE RATE OF FADING BY EXPOSURE TO LIGHT. _To face page 78._] [_Lovibond, Colour Theories._ ] The application to chemical analysis is too well known to require enlargement here. [Illustration: Fig. 9.] _Dyes._--As an example of the use of the system in the valuation of dyes, Fig. 9 illustrates the specific colour curves of four samples of Methylene Blue. No. 1 was priced at 5_s._ 9_d._ and No. 2 at 5_s._ per lb., Nos. 3 and 4 were not priced, the solutions were measured in percentages from 0·001 to 0·048 in distilled water. To find the cost per unit of colour in the priced samples is only a question of simple arithmetic, which furnishes data for the valuation of the unpriced samples. 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.