pose, the superposition of the blue and yellow tints furnished by polarized light has lately sometimes been used, but, though the result obtained is quite correct, it may be objected that this experiment was perfectly well known to Sir David Brewster, the great modern defender of the old theory, as well as to all the physicists who were his followers; and this knowledge does not seem in the least, for more than a quarter of a century, to have weakened their confidence. Nor
Fig. 2.
would it be perfectly satisfactory if I should bring about the union of blue and yellow light by the method of revolving colored disks, as is so often done; for, when we come to analyze this latter plan, we find that it consists, essentially, in presenting yellow and blue light to the eye, not simultaneously, but by a distinct succession of alternate acts. It is true that in this convenient mode of experimenting the results are the same as in that of simultaneous presentation, but just this point again would require proof, and, in a fundamental experiment like the present, ought not to be passed over in silence. To avoid these difficulties, I have contrived another plan, which will admit of our readily grasping the whole process, and inspecting its quite simple details. We have now upon the screen two large squares of blue light, and near them are two corresponding squares of yellow light (Fig. 3), and I can readily contrive matters so that the portion of the screen which is illuminated by one of the yellow squares shall also receive the light of a blue square. This we now have, and the result, as you see, is not the production of green light, or of light whose hue at all
