228 FORMATION OF IMAGES. 



to each other, and thus, as it were, to connect what would 

 otherwise be isolated facts into one systematic and harmo- 

 nious whole. 



To know facts separately, without any understanding of 

 their connection with, and bearing upon each other, or of 

 the general principles which they individually exemplify, 

 and to act only upon knowledge lying in that form in the 

 mind, is called empiricism ; but when the same facts are 

 arranged in systematic order, so that their relations to 

 each other, and the general principles which they severally 

 exemplify, are brought to view, the knowledge becomes 

 scientific. Accordingly, in what I am about to state in re- 

 spect to reflection is intended to arrange in your minds in 

 a somewhat scientific manner facts most of which, and per- 

 haps all of which, you already know in an empirical man- 

 ner. 



When any opaque surface is plane, smooth, and highly 

 polished, so that all the portions of it on which the light 

 strikes present themselves to the rays at the same angle, 

 each ray is reflected in the same manner that is, accord- 

 ing to the same law, and, after reflection, they all proceed 

 in the same directions in relation to each other^ as before, 

 though in relation to surrounding objects the direction of 

 the whole beam is turned. 



The consequence is, that light so reflected enters the eye 

 precisely in respect to the character and constitution of 

 the beam as if it had not been reflected, only it comes 

 from another direction / the object from Avliich it comes 

 of course appears of its proper form and size, and only 

 seems to be in another place. 



This is just what takes place when we see any thing re- 

 flected in a plane mirror. The engraving represents the 

 course of the rays of light in such a case, taking those em- 

 anating from one point, namely, the tip of the flame, for an 



