114 NOTES TO BOOK II. 



The Greek writers also cultivated the subject of Per- 

 spective speculatively, in mathematical treatises, as well as 

 practically, in pictures. The whole of this theory is a 

 consequence of the principle that vision takes place in 

 straight lines drawn from the object to the eye. 



The ancients were in some measure acquainted with 

 the Refraction as well as the Reflection of Light, as 

 I have noticed in Book ix. Chap. 2. The current know- 

 ledge on this subject must have been very slight and 

 confused; for it does not appear to have enabled them 

 to account for one of the simplest results of Refraction, 

 the magnifying effect of convex transparent bodies. I 

 have noticed in the passage just referred to, Seneca's 

 crude notions on this subject ; and in like manner Pto- 

 lemy in his Optics asserts that an object placed in water 

 must always appear larger than when taken out. Aristotle 

 uses the term di/a/fAacris, (Meteorol. iii. 2), but apparently 

 in a very vague manner. It is not evident that he dis- 

 tinguished Refraction from Reflection. His Commen- 

 tators however do distinguish these as Sia&acris and 

 ai>a/cXacns. See Olympiodorus in Schneider's Eclogce Phy- 

 sicce, vol. i. p. 397. And Refraction had been the sub- 

 ject of special attention among the Greek Mathematicians. 

 Archimedes had noticed (as we learn from the same 

 writer) that in certain cases, a ring which cannot be seen 

 over the edge of the empty vessel in which it is placed, 

 becomes visible when the vessel is filled with water. The 

 same fact is stated in the Optics of Euclid. We do not 

 find this fact explained in that work as we now have it : 

 but in Ptolemy's Optics the fact is explained by a flexure 

 of the visual ray : it is noticed that this flexure is dif- 

 ferent at different angles from the perpendicular, and 



