2 PROPAGATION OF LIGHT. 



them, vision was performed by something which emanated 

 from the eye to the object ; and the sense of Sight was explained 

 by the analogy of that of Touch. In this view, then, the sen- 

 sation was represented as independent of the nature of the 

 body seen ; and all objects should be visible, whether in the 

 presence of a luminous body or not. This strange hypothe- 

 sis held its ground for many centuries. The Arabian astro- 

 nomer, Alhazen, who lived in the latter part of the eleventh 

 century, seems to have been the first to refute it, and to prove 

 that the rays which constituted vision came from the object 

 to the eye. 



(3) The light of a luminous body emanates from it in all 

 directions. Thus, the light of a lamp or candle is seen in all 

 parts of a room, if nothing intervenes to intercept it ; and the 

 light of the Sun illuminates the Earth, the Planets, and their 

 satellites, in whatever position they may be placed respect- 

 ing it. 



Each physical point of a luminous body is an independent 

 source of light, and is called a luminous point. 



(4) Non-luminous bodies are distinguished into two classes, 

 according as they allow the light which falls upon them to pass 

 freely through their substance, or intercept it. Bodies of the 

 former kind are said to be .transparent ; those of the latter, 

 opaque. 



There are no bodies in nature actually corresponding to 

 these extremes. The most transparent bodies, as air and water, 

 intercept a sensible quantity of light, when of sufficient 

 thickness ; and, on the other hand, the most opaque bodies, 

 such as the metals, allow a portion of light to pass through their 

 substance, when reduced to laminse of exceeding tenuity. 



(5) In the same homogeneous medium, light is propagated 

 in right lines, whether it emanates directly from luminous 

 bodies, or is reflected from such as are non-luminous. 



