REFLECTION OF LIGHT. 391 



times be aided in this respect by extending the outspread 

 fingers between the image and the mirror. 



61O. Convex Mirrors. In convex mirrors, the 

 foci are all virtual; the images are virtual, erect and 

 smaller than their objects. The foci may be found and 

 the images determined by the means already set forth. 

 The construction is made sufficiently plain by Fig. 291. 



FIG. 291. 



Note. In constructions for carved mirrors, we have chosen two 

 particular rays for each focus sought ; one perpendicular to the 

 mirror, the other parallel to the principal axis. This was onlj for 

 the sake of convenience. Any two or more incident rajs might 

 have been taken and the direction of the reflected rajs determined 

 bj making the angle of reflection equal to the angle of incidence. 



EXERCISES. 



1. What must be the angle of incidence that the angle between 

 the incident and the reflected rays shall be a right angle ? 



2. The radius of a concave mirror is 18 inches. Determine the 

 conjugate focus for a point on the principal axis, 12 inches from 

 the mirror. 



3. (a.) Illustrate by a diagram the image of an object placed at the 

 principal focus of a concave mirror ; (b.) of one placed between 

 that focus and the mirror ; (e.) of one placed between the focus and 

 the centre of the mirror. 



