A typical example of the third type of color that does not fit in the above explanation is brown. I'll come back to it later. Hermann Helmholtz accepted and defended the theory of Jung, who became known as the Young-Helmholtz theory. Incidentally, the Helmholtz explained, finally, a phenomenon referred to in the beginning of this chapter lies in the fact that a mixture of yellow and blue colors gives the green. You can easily see how this is different from mixing yellow and blue light, by doing the following experiment, for which you need only two overhead projector and a little yellow and blue cellophane. First, attach the yellow cellophane to the lens of the projector, and blue - the lens of another and apply the projected images on top of each other. By adjusting the relative intensities, you get in the overlap zone of pure white light. This kind of mixing colors we have already seen; Then as we have explained, white light arises due to the fact that the combined effect of the yellow and blue light activates all three cones systems with the same relative efficiency as a broadband or white light. Now turn off a projector and position both filters in front of the other; you get green. To understand why this happens, we need to know that the blue cellophane absorbs longer wavelengths of light from the white portion, ie, yellow and red, and the rest, which looks blue, passes, while a yellow filter absorbs blue mainly portion and the remainder, an apparent yellow passes. The diagram in Fig. 122 shows the spectral composition of the light transmitted by each filter. Note that in both cases far from being transmitted monochromatic light. Yellow light - it does not narrow-band spectral yellow, and yellow spectral mixture with shorter green, longer orange and red waves. Similarly, the blue - a spectral blue with a dash of green and purple. Why, then, we see only yellow or only blue? The fact that the perception of yellow - the result of the stimulation of the same red and green cones without any effect on the blue cones; such stimulation can be carried out as a spectral yellow (monochromatic light with a wavelength of 580 nm), and a wider wave "smear", which is usually characteristic of pigments - just need to get width of the spectrum was not overly large and the range does not contain short waves to stimulate blue cones. Similarly, blue light spectrum has approximately the same effect as the blue plus green plus purple. When you use two filters arranged one over the other, we will get what? Both passed the filter, ie, green rays. It is in this overlap region shown in Fig. 128 graphics for broadband blue and yellow light. The same thing happens with the colors: yellow and blue paint together absorb all light except green areas, which are reflected. Note that if we had used in our experiment monochromatic yellow and siniyfiltry, placing them one over the other, they would not miss anything. Mixing occurs because light transmitted or reflected by coloring substances, it has a broadband spectral composition.