CH. XIV] 



LENSES AND THEIR ACTION 



579 



With a concave mirror it is likewise the point on the principal 

 axis where rays parallel with the principal axis before striking the 

 mirror are made to cross the principal axis after being reflected by 

 the curved face of the mirror. This point is situated half way 

 between the mirror face and the center of curvature. 



807. Conjugate foci, and the mutual relation of images. 



In figures 3 1 5-318, are shown conjugate foci on a principal and on a 

 secondary axis. In each case the object and the image might 



o, 



FIG. 316. METHOD OF IMAGE FORMATION ON THE PRINCIPAL Axis. 



In this case the object (CJ and the image (C 2 ) are equally distant from the 

 center of the lens, hence they are of the same length, and the distance between 

 them is four times the principal focus of the lens. 



FIG. 317. CONJUGATE Foci ON A SECONDARY Axis. 



The secondary axis passes through the optic center of the lens, and the con- 

 jugate focus C.^ is below the principal axis if the point C T is above it. 



change places without any change in the mutual relation of the 

 object and image. For example, if the screen picture with a magic 

 lantern were an actual scene and the magic lantern pointed toward 

 it as in projection, a small image exactly like the lantern slide 

 would be formed at the level of the lantern slide. It is from this 

 mutual relation of object and image that they are said to be 

 conjugates. 



808. How to obtain the principal focus experimentally. 

 This is accomplished by holding the lens or combination, or the 

 mirror, with the principal axis pointing directly toward the sun. 



