580 LENSES AND THEIR ACTION [Cn. XIV 



The point where the image of the sun appears indicates the prin- 

 cipal focal point, or the burning point. 



Another way to get the equivalent focal length or focus of an 

 objective is to put it in position on an optical bench like that shown 

 in fig. 159 and then to use a metric rule (fig. 178), or a lantern slide 

 of such a rule as object, and a white screen or a ground-glass on the 

 other side of the objective. The object and the screen are then 

 moved toward and from the objective until the image is of exactly 

 the same size as the object. The distance apart of the image and 

 the object is four times the focal length of the objective (fig. 316). 



FIG. 318. IMAGE FORMATION ON A SECONDARY Axis. 



With a good lens the arrows C, and C 2 are both perpendicular to ' 



the principal axis. 



C, Object. 



C 2 Image. When object and image are of the same size, as here, the image 

 is as far below the principal axis as the object is above it. 



SPHERICAL AND CHROMATIC ABERRATION AND MEANS OF CORRECT- 

 ING THESE DEFECTS 



809. Spherical aberration. By this is meant the unequal 

 bending of the light rays in different zones of a lens. As shown in 

 fig. 320, the rays passing through the outer zones of a spherical 

 lens are proportionally more bent than those which pass nearer 



808a. Equivalent focus. The term equivalent focus is often employed 

 for compound optical systems like objectives. This means simply that the 

 objective gives the same magnification or reduction in a given case as a simple 

 lens of that focus would give. 



For example, the simple lens in fig. 209, with the object 2 cm. from the 

 center of the lens, gives an image at 8 cm., four times as large as the object. 

 Now any compound system of lenses which gives a magnification of four under 

 similar conditions is said to be equivalent to this simple lens. The expression, 

 equivalent focus is frequently designated by the initial letters of the words, 

 e. f. 



