113 



NOTES TO BOOK II. 



(E.) p. 96. IT has been objected to the arrangement 

 here employed that it is not symmetrical ; and that 

 Astronomy, as being one of the Physical Sciences, ought 

 to have occupied a chapter in this Second Book, instead 

 of having a whole Book to itself (BooK in). I do not 

 pretend that the arrangement is symmetrical, and have 

 employed it only on the ground of convenience. The 

 importance and extent of the history of Astronomy are 

 such that this science could not, with a view to our 

 purposes, be made co-ordinate with Mechanics or Optics. 

 (F.) p. 108. It is proper to notice more distinctly 

 the nature of the Geometrical Propositions contained in 

 Euclid's work. The Optica contains Propositions con- 

 cerning Vision and Shadows, derived from the principle 

 that the rays of light are rectilinear : as, that the shadow 

 is greater than the object if the illuminating body be less,. 

 and vice versa. The Catoptrica contains Propositions con- 

 cerning the effects of Reflection, derived from the prin- 

 ciple that the Angles of Incidence and Reflection are 

 equal : as, that in a convex mirror the object appears con- 

 vex, and smaller than the object. We see here an exam- 

 ple of the promptitude of the Greeks in deduction. When 

 they had once obtained a knowledge of a principle, they 

 followed it to its mathematical consequences with great 

 acuteness. The subject of concave mirrors is pursued 

 further in Ptolemy's Optics* 



VOL. I. I 



