3 o8 



PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



To summarise, a concave mirror will give a mag- 

 nified view of an object, so long as the object is nearer 

 to the mirror than the centre of curvature ; when the 

 object is outside of the principal focus, the image is 

 inverted, when within the principal focus it is erect. 



2. Convex mirrors. We have seen that in con- 

 vex mirrors the foci are virtual ; hence, images will 



also be virtual. 



Let MN be a convex 

 mirror (Fig. 134),a%e'its 

 principal axis, and ab 

 an arrow in front of it. 

 Incident rays from ab 



rr. 



Fig. 134. Virtual Image of Convex 



Mirror. 



are reflected in diver- 

 gent directions, i' G H 

 and i, their backward 



prolongations meet at A and B. Here a virtual image 

 is formed, erect, but smaller than the object. Convex 

 mirrors, then, diminish the apparent size of objects. 



The size of the image may be calculated from 

 various data. Thus, the size of the image may be 

 calculated from the size of the object, if, besides, the 

 distance of each from the centre of curvature be 

 known. The formula stands thus : 



length of image 

 length of object 



distance of image from centre 

 distance of object from centre 



distance of image 

 length of image = length of object X distance of object' 



In a similar way the size of the object may be calcu- 

 lated, provided the size of the image be known, and 

 their respective distances from the centre of cur- 

 vature. 



REFRACTION OF LIGHT. 



A ray of light in passing obliquely out of one 



