COLOR PHOTOGRAPHY), in all of which three pho tographs talcen through red, green and blue filters are dyed with these respective colors and then presented simultaneously to the eye. Ives and Cros used three separate filter screens, Joly ruled a single one with lines alternately re4 blue, green, and Luiniere formed the screen directly on the sensitized plate by spreading it with a thin paste contauung . potato starch grains, some of which were stained red, some green and some blue.
The reasons for the facts in the above group are unknown. But by assuming a cer tain functioning of the retina we can form a theory which will correlate them. Like every theory this is outside of the region of fact and non-fact and is simply a mechanistic formula by means of which a whole class of phenomena can be stated in terms of a selected few. The Young-Helmholtz method of explaining. color vision (see Visiozi) hypothecates the existence of three sets of nerves in the retina: one is strongly stimulated by the red—orange end of the spectrum and less by the rest, an other most strongly by the yellow—green region and less by the ends and the third by the violet—blue end and rapidly less as we approach the red end. It is then assumed that when all three sets of nerves are acted on in about the same degree the sensation of white is produced because it is a fact that red plus green plus blue does produce white. Consider now the superposition of yellow and blue lights; according to the theory yellow stimu lates red and green because red plus green equals yellow. Therefore yellow plus blue equals red plus green plus blue equals white. This does not tell why yellow and blue cause white but it enables us to predict it without experiment. If however it can be shown di rectly or indirectly that the eye really does con tain such nerves the Young-Helmholtz theory at once enters the realm of fact. It may be in teresting to note that the whitening of linen and cotton fabrics by bluing in the laundry consists in adding hardly perceptible traces of indigo or Prussian blue to the ivory yellow tint to which they revert after being washed; it is essentially not a bleaching process.
Maxwell's color wheel affords a simple and interesting method of experimenting with ad ditive combinations. It owes its present form and use to J. C. Maxwell although it was known to the astronomer Ptolemy in the 2d century and was rediscovered by the Dutch physicist Musshenbroek in the 18th. A smooth, stiff pasteboard disc graduated at its circum ference into 25, 50 or 100 parts is mounted on the shaft of a small electric motor which can be operated by a single dry-cell. A circular piece of paper painted, for instance, half ver milion and half emerald green is placed over the sha.ft against the disc and a small wooden sleeve is then forced on so that paper and disc may turn with the axle when the motor runs. When the painted disc turns red and green are presented alternately to the eye and if the alternation is so rapid that one color occupies the place of the other before the retinal im pression of the latter has vanished then both stimuli act at the same time. The resultant color will be yellowish: not pure yellow, be cause the pigments are not monochromatic. Maxwell introduced single-colored discs, each one perforated at the centre and slit along a radius; two or more such discs can be slipped over one another and adjusted to expose to view sectors of any desired angle. They may be cut out of the Bradley colored coated-papers; a complete set will consist of the following col ors: red, orange, yellow, green, blue, violet, black, white and neutral gray and should be made in two sizes, one about twice the diam eter of the other. Dividers, into one leg of which a needle sharpened to a chisel edge is set, form a convenient instrument for cutting circles. If the motor is small the discs should not be much more than two inches in diameter, otherwise air resistance will retard the rota tion enough to produce flicker. This resistance varies as the fourth or fifth power of the di ameter depending upon whether the speed is low or high, and is therefore 16 to 32 times as much for a four-inch disc as for one half the size. The discs must be put together so that the radial edges do not catch the wind.