[ 2i6 ] [Book III. 



CHAP. V. 



OF THE PRINCIPLES OF CATOPTRICS, 



Places cf Image* in plane Refleftors.-^-tt'hy a Mirror only Half the 

 Size of an Qbjctt exhibits a perfefl Image of the Whole.- Places, 

 of Images made from Reflexion by fpherical Surfaces. -Mode of de- 

 termining the Foci of reflected Rays from fpherical Surfaces. S^xe 

 and Proportions of Images in fpherical Refleflors. Phenomena of 

 concave and convex Speculum* explained* 



BY the application of mathematical principles to 

 the few fimple fafts with which experiment fur- 

 nifties us, concerning the reflexion and refraction of 

 light, the phenomena of vifion have been reduced to a 

 fcience ; and every particular which it becomes ne- 

 cefiary to know either with refpeft to the fimple 

 effects of light on the human eye, or the ufe of glafies, 

 may be calculated by certain rules with the minuteft 

 exaftnefs. This complex fcience is called OPTICS (or 

 the fcience of vifion) and it may be fubdivided into 

 two branches, called in fcientific language catoptrics 

 and dioptrics. 



The firft of thefe relates to the theory of reflex 

 vifion, and fupplies us with rules, principles, and 

 modes of calculation, by which all the effefts refult- 

 ing from the reflexion of light may be determined 

 and explained ; and of this it will be proper to treat, 

 before we proceed to the other ftill more important 

 branch of optical fcience. 



The whole of the theory of catoptrics is founded 



upon, 



