THF PHENOMENA OF LIGHT 



339 



evident that the mirror has served to change its course, and 

 to reflect it (throw it back) into the eye where the optic nerve 

 is affected and sight a's a sensation results. Studies in 

 Physiology and Psychology as well as Physics have to do 

 with what occurs after light enters the eye, but it is the study 

 of light before it enters the eye that concerns us here. 



It is evident that the location of the image in a plane mirror 

 is determined by the light direction after reflection. The 

 image as an apparent reproduction 

 of the object is found to be located 

 back along the lines of light that 

 have come from the mirror into 

 the eye. No matter to how many 

 different eyes (people) this re- ^ 

 fleeted light from the mirror f ! \ 

 comes, it appears to originate in ! ! 

 an image of the real object that 

 has the same position behind the 

 mirror for all observers. To 



FIG. 113. An image of any 

 draw Straight lines from the eyes of point as seen in a plane mirror 



all observers back through the 



mirror to any Single point in that are equidistant from the mirror 

 ^i_ J.-L v T-.L -L i surface, and corresponding 



image means that the light which points in both are in the same 



the Corresponding perpendicular (normal) to that 

 & surface. 



originated at 



point of the object, must have 



been incident upon all parts of the mirror surface. In this 



way only could it be reflected to all observers so that the 



image of the point is at one and the same place for all. 



The complexity of this conception is much simplified by 

 conceiving that from every point in either a luminous body, or 

 of one illuminated (shining by reflected light), there is sent 

 off a continuous series of every expanding light waves whose 

 form is that of hollow concentric spherical shells. Now to 

 represent the directions in which such waves are sent out, 



