478 



CONCAVE LENSES. 



of wood or cardboard, l cm wide, which are glued, a pair to each side 

 of the box, in such a manner as to leave a space equal to the thick- 

 ness of the mirror between them ; the mirror is pushed down from 

 above into the frame thus formed, before the glass plate is fixed. 

 The box may be constructed of wood or stout cardboard ; the tubes 

 must in any case be made of cardboard. With the help of a round 

 piece of wood, 6 cm in diameter, form a tube 10 cm long, into which 

 the lens will just fit ; a second tube only 5 cm long is formed over the 

 first, so that the inner tube may move with moderate friction with- 

 in the outer one ; one end of the short tube is glued into the front 

 of the box, so that the tube may project outwards. The lens is 

 fixed in the outer extremity of the inner tube between two rings of 

 cardboard, which are formed by bending strips l cm wide and about 

 18 cm long ; one of the rings is glued into the tube so that the end of 

 the tube projects about l cm beyond the ring ; the lens is placed 

 against the edge of this ring, and pressed firmly against it by the 

 second ring, which should fit so tight into the tube as to remain firm 

 by friction alone. It will thus be possible to remove the lens again 

 easily if it is wanted for other purposes. 



The interior of the inner tube and box should be covered with rough 

 black paper. The glass plate should be covered with semi-trans- 

 parent paper ; either common paper if it is desired to copy the 

 images with pencil, or tissue paper of the kind used for the screen 

 in Art. 38, if the images are to be merely looked at. 



Concave lenses may be considered as composed of a 

 number of small prisms which have their refracting 

 edges turned towards the middle of the lens, in the same 

 manner in which convex lenses may be considered as 

 made up of prisms which turn their refracting edges away 

 from the centre. It follows that concave lenses will 

 refract light so as to cause it to diverge from the axis 

 of the lens. Light incident upon a concave lens parallel 

 to the axis is refracted so as to diverge, after passing 

 through the lens, as if the light proceeded from a point 

 /, fig. 275. This point is the principal focus of the lens, 

 and may be designated as the point of divergence or 

 negative focus ; its distance from the lens is the focal 



