For gold-fish farmers at Koriyama near Nara, so-called “Mizutsukuri” (“water-making”) is an important but arduous daily task so as to maintain the pond water containing adequate quantity of phytoplankton, upon which the gold fish production depends. The decision of the suitability of “Mizutsukuri” is always given by the gradation of color of pond water derived from phytoplankton which has increased in adding sufficient fertilizers. FOREL-ULE'S color standards widely used in limnology is not suitable for the indication of the color grades of pond water, because they express only hue of the three values in GRASSMAN'S law of color language which is now adopted internationally (Commission Internationale de l'Eclaisage). Therefore, the writers have developed a new easy field method which can indicate the three values of the C. I. E. color language in combination with the principle of “additive color mixture” by using a MAXWELL'S disc.
In studying by this method of about eighty gold-fish ponds, at Koriyama the writers have succeeded in representing all the water colors of them in mixing the color-discs of yellowish green, orange and black which have spectral reflectance curves as shown in Fig. 3. Table 1 shows the tristimulus value, chromatic coefficient, wavelength and excitation purity of three color discs. The apparatus used is as follows : Each disc has a radial slit extending from the center hole to the circumference so that three discs, when put upon with each other, may be intermingled with a portion of each which remains visible. When such discs are rotated by a miniature motor connected to two 1. 5 v dry cells producing a speed of more than 35 times per second, they are visible as an individual color. The color resulting from this mixture depends upon the relative amount of the exposed areas of the discs used. In order to compare the mixed paper color with the color of pond water under the same condition, a gray-colored paper mask with two windows of the same size and form is used (Fig. 4). Table 2 indicates the water colors of 30 gold-fish ponds which were selected at random in September, 1954. The chromaticity positions of each water color thus obtained were given on the C. I. E. diagram in Fig. 5 in using each chromatic coefficient in Table 2.
The results obtained on the water colors of gold-fish ponds are as follows :
1). The wavelengths are distributed between 560 mμ, and 590 mμ, mostly between 560 mμ, and 570 mμ, (Table 3).
2). The brightness is distributed uniformly between 4.0 and 16.0 per cent (Table 4).
3). It is remarkable that about 70 per cent of the ponds examined has given excitation purity at the values between 30 and 40 per cent (Table 5).
4). It was obtained a positive correlation between the brightness and the excitation purity, as shown in the graph of Fig. 6 and as denoted as Y= 0.279, Pe +0.05, and the correlation coefficient (r) being 0.763.
In the waters which have become extremely eutrophic like the goldfish ponds under consideration, the factors deciding the water colors are the quantity of phytoplankton which consists chiefly of blue-green and green algae especially
Microcystis and
Scenedesmus in dominancy throughout the year. If the water color be investigated in such a relation as mentioned above, the significance of each value of the wavelengths, the brightness and the excitation would become clear, and the method discussed would be able to apply to practical purposes.
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