The box used for this purpose is illustrated in Fig. 3, and has one surface covered with standard white for the purpose of easy comparison with the pigments. The standard black aperture (1) is in the middle. The pigmentary blacks (2 to 10) are arranged over this, and the pigmentary whites and greys (11 to 20) underneath, each being numbered in accord with its intensity as tabulated. [Illustration: Fig. 3.] The degrees of blackness are the number of absorptive units required to reduce the standard white to equal the pigments in each case. Light Absorbed by Various Pigments. ---+---------------------+--------+----------+------- No.| |Absorbed|Unabsorbed|Initial | | Light. | Light. | Light. ---+---------------------+--------+----------+------- 1 |Black Hole in Box | 36 | -- | 36 2 |Optical Black | 20 | 16 | 36 3 |Lamp Black | 17 | 19 | 36 4 |Vegetable Black A | 17 | 19 | 36 5 | " " B | 14 | 22 | 36 6 | " " C | 15 | 21 | 36 7 |Indian Ink on Paper | 14 | 22 | 36 8 | " Solid | 12 | 24 | 36 9 |Boot Black | 11 | 25 | 36 10 |Black Lead | 9 | 27 | 36 ---+---------------------+--------+----------+------- This gives a working scale of colourless light intensities, the terminals being black and white, with a range of 36 units. _The Standard White._--White is the natural terminal of the luminous end of the scale, and it is necessary to select a physical objective white as a constant. Pure precipitated lime sulphate has been adopted, and departures from the light intensity of this are recorded in units of lessened light intensity throughout the scale, comprising all degrees of colourless whites, greys, and blacks. Strictly speaking, white is a qualitative term only, until the degree of variation from the zero of the scale has been established. The measured variation then takes its position in the scale of luminous intensities according to its numerical unit value. Light Absorbed by Various White and Grey Pigments. ---+--------------+--------+---------+------- No.| Pigments. |Absorbed|Reflected|Initial | | Light. | Light. | Light. ---+--------------|--------+---------+------- 11| Grey Paint E | 6·0 | 30·0 | 36 12| " D | 5·0 | 31·0 | 36 13| " C | 4·0 | 32·0 | 36 14| " B | 2·0 | 34·0 | 36 15| White Paint A| 0·7 | 35·3 | 36 16| White Paper D| 0·3 | 35·7 | 36 17| " C| 0·2 | 35·8 | 36 18| " B| 0·25 | 35·75 | 36 19| " A| 0·15 | 35·85 | 36 20| Chinese White| 0·006 | 35·994| 36 ---+--------------+--------+---------+-------