A triangular solid piece of glass, on which if a ray of light be cast it will be distinctly divided into the seven colors we see in a rainbow. By this fact we see that white light is composed of different rays which have different reflective susceptibilities.
What is a Spectrum?
It is this beautiful band of seven colors obtained by the refraction of a ray of light through the prism.
Whence come the colors in the objects we see in nature?
They all come from light; every object has a power to absorb certain rays and to reflect others. A red cloth, for example, absorbs all the other colored rays except red, and this it gives off, thus appearing red.
Why are the leaves of plants green?
Because a peculiar chemical substance called chlorophyl, formed within their cells, absorbs all other rays of light, reflecting only blue and yellow—which mixture produces the different green tints.
What is Photography?
The word means "light drawing." It is a mode of fixing on certain substances the lights and shades of any object by means of a lens inserted in a camera obscura. This process was first called Daguerreotype from the name of the inventor, Daguerre. A plate of copper thinly coated with silver is exposed to the vapor of iodine, then placed in a camera obscura, where an image of the object to be presented through a lens is cast upon it. Ambrotype is the same application to glass. There are now different variations of method in the use of the same agents. Now photography consists in taking the images on what is called a negative—that is, a glass coated with a silvered collodion (gun-cotton dissolved in alcohol and ether) film. From this plate another image is taken on silvered paper, which we call the positive image. There are also other chemicals used instead of silver.