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MOTION
PICTURE PHOTOGRAPHY
or turned to one side by the glass ; that is, the light was refracted. This refraction only takes place at the point of entrance between two mediums of different density. After being refracted at the surface the light continues to travel through the second medium in a straight line from the point of entry to the point where it emerges on the other side where a second refraction takes place, light again continuing to travel in a straight line. This angle of refraction varies according to the density of the medium in its relation to light and is always the same in the same medium, thus different kinds of glass and all transparent crystals and liquids have different angles of refraction. This angle of refraction is
Illustrating the relationship between lenses and prisms. If we
consider a lens as consisting of innumerable small prisms built up
around a comimon center this relationship will become apparent.
called the index of refraction. These indexes of refraction have been measured by mathematicians who make calculations for manufacturers of lens and predict all of its properties before one has been made. Such calculations are, however, far beyond the scope or needs of any ordinary photographer.
In Figure No. lo, we have a point from which emanates rays of light. Suppose we take a number of prisms with varying angles as illustrated in the diagram, the angle of each being such that each ray which passes through each prism is refracted to the point so that each of these rays is again collected at this point. Let us now examine the line of prisms which we have thus placed. The central prisms have sides which are nearly parallel, which progress outward from the center, the angle increases until the two faces come together. We will now replace the line of prisms
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