MAGNirYING POWEES. 



159 



it will be i^equisite, in the first place, that certain terms, 

 such as spherical and chromatic aberration, be fuUy under- 

 stood. Most persons are familiar with the fact, that when 

 parallel rays of light fall upon a piano or double convex 

 lens, they are brought to a point at a certain distance 

 from the lens, which point is termed their focus. Thus sup- 

 pose in fig. Ill that the rays, L L, &c, which are drawn 

 parallel, are, after passing through the planoconvex lens, 

 brought to a focus at F, this would take place if the lens were 

 perfect ; but it is found in practice, that instead of meeting in 

 a single point, F, the rays are subject to two different causes 

 of error or aberration, the first is called the spherical, the 

 second the chromatic aberration ; and as no lens can be made 

 except that seen in section in figs. Ill and 112, viz., the plano- 

 convex, without both its surfaces being portions of spheres, 

 and it having been abundantly proved, that no lens with a 

 spherical surface can bring the rays of light issuing 

 from one point into a focus at another point, all must 



be subject to what is 

 termed spherical aberra- 

 tion, as shown in fig. 112, 

 where L L, &c., repre- 

 sent five parallel rays of 

 light, the two outer of 

 which win be brought 

 into a focus at F, whilst 

 the two next, supposing 

 them very near the cen- 

 tre, will be refracted to 

 a more distant point at f, 

 the distance from F to 

 / being called the longi- 

 tudinal spherical aberra- 

 tion. If the lens were 

 placed with its convex 

 side towards the parallel 

 rays, the aberration would then be comparatively trifling ; the 

 same result would be obtained even if the lens were equally 



Fig. 111. 



Fig. 112. 



