418 THE OUTGO OF ENERGY 



The principal focal distance of a double con- 

 vex lens is approximately equal to the radius 

 of curvature. 



Conjugate Foci. Place in the lantern the dia- 

 phragm of 2 mm. aperture. Be move the tubes 

 holding the projecting lenses. Place the convex 

 lens against the window of the optical box. 

 Place the black screen twice the focal distance 

 from the lens. Move back the lantern until a 

 clear image of the luminous aperture appears 

 on the screen. 



The point from which rays passing through 

 a lens diverge, and the point to which they con- 

 verge, are termed conjugate foci. Measure the 

 distance of the luminous aperture from the lens. 

 It will be found to be twice the focal distance. 

 When the point of divergence is separated from 

 the lens by twice the focal distance, the point 

 of convergence is equally distant from the other 



of the object from the principal surface of the lens (see page 46); 

 then-^- 4- -= j. The relation between the size of the image 



and the size of the object is L \ I : : A I a ; then - = , 

 and, by substitution, = + , whence / = A 



/ A ^ AV L+T 



Compared with the thickness of the lens, the distance of the 

 object from the lens is so great that it may be used in place of 

 the unknown distance from the principal surface (Kohlrauschr 

 Leitfaden der praktischen Physik, 1887, p. 142). 



