A beam of the brilliant light from a “limelight," closely resembling that of the sun, is here being thrown through a triangular glass prism and magnifying glass on to a white screen. The effect of the prism is to break up the light into the series of colours, called the “spectrum," of which white light is composed. The colours are seven in number—violet, indigo, blue, green, yellow, orange, red.
The artificial word VIBGYOR will help the reader to member their order. The separation is due to rays of different colours being bent out of their original course in different degrees—violet most, read least. This particular spectrum, called the solar spectrum, is peculiar to sunlight of the same nature.
Flames of various kinds produce distinctive spectra, so that the trained observer can tell what a substance is composed of by burning it in a gas flame and examining its spectrum by means of the spectroscope, an instrument containing a glass prism.
The spectroscope even enables the astronomer to determine of what stars many millions of miles away are made. (R.H. Poole)
HOW THE CAMERA HELPS IN REPRODUCING COLOURS
Printers made use of the fact that the three primary colours of the spectrum—yellow, red and blue—can be mixed to make any other colour.
In C we have a three-colour reproduction of a famous painting. To reproduce this picture as shown in C three negatives of the picture are made with the camera, through colour " filters," which cut out respectively yellow, red and blue rays.
Positive printing blocks are made from the negatives, and each is covered with ink of the primary colour, which did not affect its negative.
Printed separately, the “yellow " " red " and " blue " blocks would produce prints A, B and D respectively. In E red is printed on the top of yellow; and in C blue has been superimposed on E, giving a “three-colour “print in the actual colours of the original. (R.H. Poole)
FOUR COLOUR PRINTING
Mix up three primary colour can produce secondary color, for example, red and green yields yellow, red and blue yields magenta (a purple hue), and green and blue yield cyan (a turquoise hue). In modern printing, four inks are used: three secondary colors plus black. These ink colors are cyan, magenta and yellow; abbreviated as CMYK. (Wikipedia)
R.H. Poole, e. News’ Pictorial Knowledge. London: George newnes Limited.
Wikipedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Color_printing