ABSTRACT
When a light wave is incident upon a solid or liquid body, the direction in which it travels is, in general, changed considerably. A portion of the incident light is reflected from the surface of the body, another portion penetrates into the inside of the body where it is absorbed, whilst the rest eventually appears at the other side of the body if this is transparent. The magnitude of one or other of these components varies from one body to another, and, moreover, it depends upon the wave-length of the incident light. The light energy is now sufficient to stimulate the optic nerve of our eye. It is well to notice, however, that we only discern the position of the star and learn that it really does exist, but it is not a true observation in which we recognize the details or shape of the object viewed.
