Light 187 



As the sun shines through a raindrop, the light is broken up 

 into its colors, as in a triangular prism. Each drop shows all 

 of the colors, but at different angles. A drop high in the air may 

 send red waves directly to your eyes, the other colors passing 

 overhead. A lower drop is meanwhile sending you violet, and 

 other drops, between the two, supply the remaining colors of the 

 spectrum. The thousands of falling raindrops thus give a con- 

 tinuous spectrum across the sky. Sometimes the light strikes 

 a higher set of raindrops, and by double refraction in these 

 drops causes a second rainbow, above the first, and with the 

 colors reversed. 



Lenses. A lens is a piece of transparent glass so made that 

 its surfaces are not parallel, and at least one is curved. It is used 

 for bending light rays. One lens may be of such shape that it 

 will spread the rays from a beam. Another lens may bring the 

 rays closer together, in which case it is said to focus them. This 

 means that if they are allowed to continue without interruption 

 the light rays will come together at some point known as a focus. 

 Lenses are made in many different shapes. 



Lenses may be divided into two classes. Convex lenses are 

 thicker in the center, and tend to bring the light to a point. 

 Concave lenses are thinner in the center and tend to spread the 

 light. The most common use we have for these two forms of 

 lenses is in making eyeglasses. The convex lens is also used in 

 the camera and in the projection lantern. 



If a beam of light is allowed to pass through a darkened 

 room, from a hole in the window shade or from a projection 

 lantern, it may be seen as dust particles in its path become 

 illuminated. This effect can be increased by tapping a black- 

 board eraser in the light streak to add dust particles. If lenses 

 of different shapes are held in this beam of light their effects 

 may be noticed, either by observing the changes in the beam, 

 or in the light spot on a cardboard held in position back of 

 the lens. 



Sight. The eye is the organ of sight. Its parts are the 

 cornea, the sclerotic coat, the iris, the pupil, the lens, the retina 

 and the choroid coat. The three most essential parts are the iris, 



