66 PHENOMENA OF LIGHT AND COLOUE. 



of refraction until about the year a.d. 1100, when Alhazen 

 attempted to determine the relation between the angles of 

 incidence and refraction, and set out some of the laws of 

 refraction.* 



Aristotle was acquainted with the phenomenon now called 

 phosphorescence, but did not understand it. He says that it 

 is the nature of smooth surfaces to shine in the dark, but 

 yet they do not produce light. t Again, he says that some 

 objects are seen in the dark, e.g., those which seem to be of 

 the nature of fire and shining, such as, for example, fungi, 

 horn, sepia juice, and the heads, scales, and eyes of fishes, 

 and that these do not show the proper colours of the objects 

 themselves. 1 



It is not clear what is meant by the assertion that light 

 {(pSii) is not produced by objects shining in the dark. If 

 Aristotle had said that heat is not produced, he would have 

 made a substantially true statement, but light is produced 

 and some phosphorescent bodies emit a light as brilliant as 

 that given out by firebrick or other ordinary substances 

 heated to a high temperature. It is clear, from the passage 

 in De Anima, ii. c. 7, cited above, that Aristotle did not 

 consider phosphorescent bodies to be actually of the nature 

 of fire, in which case they would emit heat, like a flame or 

 incandescent body, and this may be a reason why he states 

 that no light is emitted, since no heat accompanies the 

 shining effects. That a phosphorescent body does not shine 

 with a colour like that of the body itself, as Aristotle says, 

 is true, e.g., the white flesh of fishes often shines with a 

 delicate green light. 



Difficult though it is to follow Aristotle's speculations on 

 light, it is more difficult to follow his speculations on colour. 

 It has been stated already that, according to him, the 

 Diaphanous exists in varying degrees in all bodies. He 

 defines the colour of a body to be the boundary of the 

 Diaphanous which is in the body.§ Whatever the nature 

 of the Diaphanous may be, it cannot exist separately, 

 Aristotle says, but has limits to the same extent as the 

 bodies in which it exists. Light exists in the Diaphanous, 

 but, if a particular body be considered, it is evident that the 



=!= OpticcB Thesaurus Alhazeni Arahis, F. Risnerus, Basle, 1572, Book 

 vii., especially c. 3, entitled " De qualitate refractionis lucis in corporibus 

 diaphanis." -\ De Sensu, i^c, ii. 4o7rt. 



I De Anima, ii. c. 7, 419a; De Sensu, dc, ii. 4376. 



§ De Sensu, dc, iii. 439a and b. 



