468 



REFRACTION THROUGH LENSES. 



and one concave surface, but the convexity exceeds the 

 concavity; such a lens is called a 'meniscus/ and it 

 shares with the two other kinds the property that it is 

 thickest in the middle. 



Concave lenses are ' diverging/ that is, rays of light 

 diverge after being refracted by them. Fig. 267 d is a 



FIG. 267. 



4 double concave ' lens, being bounded by two concave 

 surfaces ; e is a ' plano-concave,' and / a concavo-convex 

 lens, in which the concavity exceeds the convexity. All 

 diverging lenses are thinnest in the middle. 



The straight line drawn through the centre of the 

 lens perpendicular to both surfaces is called the 'axis;' 

 the axis obviously passes through the two centres of 

 curvature of the bounding surfaces. 



In fig. 265 only the seven rays actually represented 

 could possibly intersect, after refraction, in the point c ; 

 for a ray from a, which falls upon the small prism b 

 near the upper edge, would pass after refraction above 

 c, while another ray which falls upon the same prism 

 nearer to its base would pass after refraction below c. 

 In order to refract these rays also towards c, the upper 

 ray would need to be somewhat more, and the lower ray 

 somewhat less refracted, than the middle one, or the 



