IMAGES AND SHADOWS. 



667 



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 na- 

 ture, 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 im- 

 ages of things we have to deal with. We know the external world in 

 its distances, forms, and colors, because its visible objects are all du- 

 plicated in the eye. The cloud, the landscape, the cathedral, that 

 excites our thought and kindles our feeling, is, in each case, but a pict- 

 ure recreated from the external object by the agency of light. 



Fig. 3. 



Inverted Image op Landscape. 



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 accu- 

 rately 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 



