62 PHENOMENA OF LIGHT AND COLOUR 



not fire nor any material substance, nor, consequently, is it 

 an emission from a material substance,* and that the theory 

 that sight is due to something which issues from the eye, 

 and is capable of extending as far as the stars, or, as some 

 say, that it is due to something which issues from the eye 

 and meets with something issuing from the object, is 

 altogether absurd, t Aristotle's own views on the nature 

 of light seem to have been as follows : Something which 

 he calls the Diaphanous (to ^ia(pu.vk) is present not only in 

 air, water, and other transparent substances, but also, in 

 varying degrees, in other bodies. It is not capable of 

 separate existence, being a kind of property and power 

 common to all bodies, and, when excited by the presence in 

 it of something of the nature of fire, light is produced, 

 while the absence of anything of the nature of fire results 

 in darkness.! Light is the energy of the Diaphanous, and 

 is, as it were, the colour of the Diaphanous, when this is 

 in actual or full existence (£VT£^e%£Ja) through the influence 

 of fire or something of this kind, such as, for example, the 

 upper body.§ The upper body, referred to here, is the 

 Aristotelian asther, which resembles the aether of modern 

 scientists in some respects, but is here supposed by Aristotle 

 to be an exciting cause of light. 



The Diaphanous was evidently passive, but capable of 

 being influenced by fire or something of the nature of fire. 

 The relationship between fire or the like and the Dia- 

 phanous seems to be like that between form and material, 

 as exemplified by a stone statue, for, when the Diaphanous 

 is modified by the presence of fire or the like, light is 

 produced, while the stone, modified so as to be of a par- 

 ticular form, is a statue. 



In an important passage Aristotle says : — " I have stated 

 in other books that sight is impossible without light, but 

 whether it is light or air which intervenes between the 

 object and the eye, it is the motion through this medium 

 that causes sight." |i 



This may seem to foreshadow the undulatory theory of 

 light. It seems, however, from other passages that the 

 motion was not an undulatory one, although he nowhere 

 seems to explain what kind of motion he meant. He says 

 that odours and sounds travel through a medium before 

 they cause sensation, and that Empedocles believed that 



* De Anima, ii. c. 7, 4186. f De Sensu, dc, ii. 438a, I Ibid. in. 439a. 

 § De Anima, ii. c. 7, 4186. || De Sensu, dtc, ii. 4386. 



