SENSE OF SIGHT. 853 



Refraction takes place not only through media with plane surfaces, 

 but likewise through media bounded by curved surfaces, for the circum- 

 ference of a circle may be supposed to be made up of a number of infinitely 

 small, straight lines : this is indicated in the case of a lens in Fig. 371. 

 Rays of light passing through a double convex lens in passing in are bent 

 toward the perpendicular (Fig. 372). Now, if the rays thus acted upon 

 be followed they will be found to meet at a point on the opposite side 

 of the lens, called the focus, at which light and heat rays will converge. 

 Rays of light striking the centre of curvature of both surfaces of the lens 

 will pass through unchanged. Such a line is called the chief axis, and 

 the centre of this line is the optical centre of the lens. Rays passing 

 through the optical centre are termed principal or chief rays. Rays 

 parallel with the principal axis of the lens are refracted so that they are 

 collected on the opposite side of the lens at a point called the principal 

 focus ; the distance of this point from the central point of the lens is 

 called the focal distance. On the other hand, it is evident that rays 

 diverging from a luminous point at the principal focus will be so refracted 



Fig. 373— Diagram illustrating Action of a Double Convex Lens of 



High Curvature on Divergent Rays, [(ranot.) 



The divergent rays from the luminous point, U are brought to a focus at I behind F, the principal focus 



at which parallel rays, S B, converge. 



as to be parallel when they pass from the lens. Again, rays of light in the 

 principal axis and from a point beyond the principal focus will converge 

 to a point on the opposite side of the lens (Fig. 373). 



Four cases are possible : First, when the distance of the light from the 

 lens is equal to the focal distance the focus will lie at the same distance 

 on the opposite side of the lens, i.e., twice the focal distance; second, 

 if the luminous body approach to the lens, or, what is the same thing, to 

 the focus, then the focal point is moved farther away; third, if the light 

 is still farther from the lens than twice the focal distance, then the focal 

 point comes correspondingly nearer to the lens ; fourth, if the rays 

 proceed from a point on the chief axis within the focal distance they 

 will divero-e on the opposite side of the lens and not again come to a 

 focus ; while, on the other hand, converging rays passing through a convex 

 lens will have their focal distances at a nearer point than that at which 

 parallel rays are collected. These facts are illustrated in the following 

 diagrams (Fig. 374). 



