496 THE HUMAN BODY. 



verging lens there is such a point behind it at which the 

 rays from a given point in front of it meet: the point of 

 meeting is called the conjugate focus of the point from 

 which the rays start. If instead of a luminous point a 

 luminous object be placed in front of the lens an image of 

 the object will be formed at a certain distance behind it, for 

 all rays proceeding from one point of the object will meet 

 in the conjugate focus of that point behind. The image 

 is inverted, as can be readily seen from Fig. 129. All rays 



from the point A of the object 

 meet at the point a of the 

 image; those from B at b, and 

 those from intermediate points 

 at intermediate positions. If 

 the single lens were replaced 

 by several combined so as to 



converging lens. form an optica l sys tem the 



general result would be the same, provided the system 

 were thicker in the centre than at the periphery. 



The Camera Obscura, as used by photographers, is an 

 instrument which serves to illustrate the formation of 

 images by converging systems of lenses. It consists of a 

 box blackened inside and having on its front face a tube 

 containing the lenses; the posterior wall is made of ground 

 glass. J[f the front of the instrument be directed on ex- 

 terior objects, inverted and diminished images of them will 

 be formed on the ground glass; those images are only well 

 denned, at any one time, which are at such a distance in 

 front of the instrument that the conjugate foci of points 

 on them fall exactly on the glass behind the lens: objects 

 nearer or farther off give confused. and indistinct images; 

 but by altering the distance between the lenses and the 

 ground glass, in common language "focusing the instru- 

 ment," either can be made distinct. For neajLoMg cts ^ e 

 lenses muaLJia-JEarther from the surface on which the 

 image is to be received, and for distant nearer. The 

 reason of this may readily be seen from .Fig. 130. If the 

 system of lenses brings the parallel rays a c and I d, pro- 

 ceeding from an infinitely distant object, to a focus at x, 



