sions. Touch, taste, smell, and hearing, are the senses by which he becomes acquainted with many properties of things immediately around him; but the universe is reported to him through the sense of vision and by the agency of light. Somehow, in their mysterious nature, the luminous rays from all sources, distant and remote, effect a change in the nerve-structure of the eye by which impressions are transmitted to the brain. Into that mode of action we cannot now enter, but will confine our attention at present to the property of light by which luminous images are produced. For it is, after all, the images of things we have to deal with. We know the external world in its distances, forms, and colors, because its visible objects are all duplicated in the eye. The cloud, the landscape, the cathedral, that excites our thought and kindles our feeling, is, in each case, but a picture recreated from the external object by the agency of light.
The first property, or law, of light, upon which the production of images depends, is simply that it moves in straight lines through any uniform medium that it can traverse. We are all familiar with the general fact that the path of light is rectilinear, but it may be accurately proved by a very simple experiment.
Two screens, A, B (Fig. 1), each pierced with a minute hole, are so arranged that the apertures are in a line with the flame of a candle, C. An eye, placed in this line behind the screens, is then able to see