222 THE HUMAN BODY 



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 proceed- 

 ing 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. 86. 



. Diagram illustrat- AIT rnv<J f rnrn fh rinint A nf thp nhippt 

 ing the formation of an image Ali ra y S Lt A DDject 



by a convex lens. 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 form an optical system the general result would be the 

 same, provided the system were thicker in the center than at the 

 periphery. 



A moment's consideration of the diagram (Fig. 86) shows us 

 that the nearer any luminous point is to the lens the further be- 

 hind the lens its conjugate focus will be. The rays from near 

 points are more divergent when they strike the lens than are those 

 from far points, they are therefore not so much bent toward each 

 other upon emerging, and their point of meeting is further back. 

 There must be some point near the lens from which rays are so 

 divergent that after emerging they do not meet at all, but con- 

 tinue to diverge or form a parallel beam. A plane so located with 

 reference to a lens that rays from any point in it striking the lens 

 emerge in a parallel beam is the principal focal plane of the lens. 

 The thicker a lens the nearer to it is its principal focal plane. 



The Ordinary Photographic Camera is an instrument which 

 serves to illustrate the formation of images by converging sys- 

 tems 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. If the front of the instrument be di- 

 rected on exterior objects, inverted and diminished images of 

 them will be formed on the ground glass; those images only are 

 well defined, 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 dis- 



