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EXPERIMENTAL PHYSIOLOGY. 



to come, not from the actual object itself, but from some other point 

 known as the IMAGE of that object. Each point on a luminous 

 object has a point image corresponding to it. The position of the 

 image of the whole object may be found by defining the position 

 of the image of each of its extreme or limiting points. To find these 

 one applies the LAWS OF REFLECTION, which state (1) that the inci- 

 dent ray (that is, the ray before reflection), the reflected ray, and 

 the perpendicular to the surface at the point of incidence, are all 

 in one plane, and (2) that the angle between the incident ray and 



FIG. 37. To illustrate the formation of an image by a plane mirror. 



the perpendicular (or the ANGLE OF INCIDENCE) is equal to that 

 between the perpendicular and the reflected ray (the ANGLE OF 

 REFLECTION). Fig. 37 shows the construction for a plane mirror. 

 PQ is the object, PA and PB, incident rays, and AE and BF, the 

 perpendiculars to the surface at the points of incidence. These, 

 together with the reflected rays, AC and BD, are all in the plane 

 of the paper, i and i' are the angles of incidence. After reflection 

 the rays AC and BD pass in such a direction as to make each 

 angle of reflection equal to the corresponding angle of incidence 

 (r = i, r' = i'.) These rays both appear to come from a single point 



