PHENOMENA OF LIGHT AND COLOUE. 63 



sunlight had to travel through a medium before it reached 

 the eye, but, about light, he adds, a different account must 

 be given, for light is due to the existence of something in 

 the medium, and is not amotion.* This last statement 

 causes some difficulty, because it seems to be inconsistent 

 with the passage in De Sensu, d-c, ii. 4386. The word 

 Kivrryii, used in De Sensu, dc, vi. 4466, is a general one for 

 "motion," and does not give much assistance in ascertaining 

 what Aristotle meant. The context, however, indicates 

 that the meaning is that light is not due to a motion of 

 translation, necessarily taking place during an interval of 

 time. In fact, Aristotle says, in De Seiisii, d-c, vi. 441 a, 

 that it is reasonable to believe that, when there is a medium 

 between a sensory organ and an object of sensation, the 

 effects are not all produced on the sensory organ at the 

 same time, except in the case of light and sight. 



Aristotle was not the first to introduce the idea of a 

 motion of the medium between the eye and the object seen 

 by it. Democritus believed that the emanations from the 

 object did not reach the eye, but set in motion the inter- 

 vening air. 



Like many other ancient philosophers, Aristotle was 

 aware that light should be treated as if it were propagated in 

 straight lines. Many parts of his descriptions of optical 

 phenomena, e.g., rainlDOWs and eclipses, show this, and some 

 questions are proposed in the Aristotelian work called the 

 Problems, the answers to which depend on the assumption 

 that the propagation of light is in straight lines. One of 

 these questions is particularly interesting, and asks why sun- 

 light shining through apertures bounded by straight lines 

 does not form rectilinear images but circular ones. The 

 first part of the answer suggests that it may be that the 

 light is propagated in conical form and, the base of a cone 

 being circular, the images are circular also. Then follows 

 an explanation which is quite Aristotelian, and depends on 

 an assumed inability of visual rays, which are few and weak, 

 to reach the object to be seen ; such an assumption is made 

 in other places by Aristotle, particularly in his explanation 

 of rainbows. The rays of light, passing through the corners 

 of the apertures, being assumed to be few and weak, are not 

 effective, but only the rays passing through the central 

 parts, these rays being assumed to be numerous and strong ; 



* De Sensu, dc, vi. 446rt and b. 



