746 



ACTION OF A CONVEX LENS. 



in the second medium i Prolong the unrefracted ray, P, *, Q parallel to *, y, then y, Q is the 

 direction of the refracted ray (according to 2). 



2. Construction of the image for a given object In fig 



529, B, c, d, a, b, k, p, and ft, 

 C, D are as before. Suppose 

 a luminous point (o) in the first 

 medium, what is the position of 

 the image in the second me- 

 dium ? Prolong the unre- 

 fracted ray (o, k, P), and draw 

 the ray (o, x) parallel to the axis 

 A (a, &)." The parallel rays (a, e 

 and o, x) reunite in p (accord- 

 ing to proposition 1). Prolong 

 x, ft until it intersects the ray 

 (o, P), then the image of o is 

 at P, the rays of light (o x and 

 o k) proceeding from the lumi- 

 Fig. 528. nous point (o) reunite in P. 



Construction of the refracted ray and the image in several refractive media.-If several 

 refractive- media be placed behind each other, we must proceed from medium to medium with 



'I--";;;;;--;: jjjjjjj I 



. Fig. 529. P 1 



the same methods as above described. This would be very tedious, especially when dealing 

 with small objects. Gauss (1840) calculated that in such cases the method of construction is 



II. 



Fig. 530. 

 very simple. If the several media are "centred," i.e., if all have the same optic axis, then the 

 refractive indices of such a centred system may be represented by two equal, strong, refractive 

 surfaces at a certain distance. The rays falling upon the first surface are not refracted by it, 



