A. They are refracted below the horizon, and are soon made invisible by the curve of the earth. (See last figure.)
Q. Why are the edges of clouds more luminous than their centres?
A. Because the body of vapour is thinnest at the edges of the clouds.
Q. What is the cause of morning and evening twilight?
A. When the sun is below the horizon, the rays (which strike upon the atmosphere or clouds) are bent down towards the earth, and produce a little light called twilight.
See figure on p. [399].—Here the rays of PA will give some light.
Q. Why is a ray of light composed of various colours?
A. If solar light were of one colour only, all objects would appear of that one colour (or else black.)
Q. Why are some things of one colour, and some of another?
A. As every ray of light is composed of all the colours of the rainbow, some things reflect one of these colours, and some another.