=409. Three-color Printing.=--Since all colors may be produced by mixing the three colors, light red, green, and blue-violet, these are called _the three primary colors_. The so-called primary pigments or paints are simply the complements of the three primary colors. They are, in order, peacock blue, crimson, and light yellow. The three pigments when mixed yield black, since combined they absorb all kinds of visible light. The process of three-color printing, now so generally employed in printing colored pictures for books, calendars, etc., consists in combining upon white paper three colored impressions, using successively the three primary pigments (yellow, crimson and blue) from plates prepared as follows: Three photographs of a given colored object are taken, each through a different sheet of gelatine called a filter, stained the color of one of the primary colors. From these photographs half-tone blocks are made in the usual way. The colored picture is made by carefully superposing impressions from these blocks, using in each case an ink whose color is the complement of the "filter" through which the original picture was taken. An illustration of the process is given upon the plate in the frontispiece of this book. Important Topics 1. Color, due to wave length; dispersion by prism, sphere in rainbow, complementary colors, color of opaque and transparent bodies. 2. Spectra, solar; formation of rainbow; bright-line spectra, how formed, how used; dark-line, how formed, used. 3. Theory of color vision. Three color printing. Exercises 1. How does a white flower look when viewed through a blue glass? Through a red glass? Through a red and blue glass at the same time? 2. Why does a red ribbon appear black when seen by blue light and red when seen by red light? 3. In what part of the sky must you look to see a rainbow in the morning? In the afternoon? Explain. 4. How would you arrange two similar prisms so as to produce double the deviation produced by one? 5. The color of an object depends upon what two things? 6. What kind of a spectrum should moonlight give? Why? 7. A mixture of green and red lights gives a sensation of yellow. Can you suggest why a mixture of blue and yellow lights gives the sensation of white? (8) NATURE OF LIGHT, INTERFERENCE, POLARIZATION =410. The Corpuscular Theory.=--The theory of the nature of light that was most generally accepted until about the year 1800, held that light consists of streams of minute particles, called corpuscles, moving at enormous velocities. This _corpuscular theory_ was in accord with the facts of reflection and the _rectilinear_ motion of light, but was abandoned after the discovery of the _interference of light_, as it could not account for the latter phenomenon.