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there is a corresponding focal point in the image, and 

 therefore the image will be a perfect facsimile of the 

 object. 



Observe, then, (i) the image is inverted. All images 

 made by lenses (dioptric images) are inverted. They 

 must be, because the central ray of the pencil from each 

 radiant passes straight through the lens without bend- 

 ing, and therefore these central rays all cross one another 



Fig. 73. 



at a certain point in the lens called the nodal point. Ob- 

 serve again (2) that in order to have a sharp image the 

 receiving screen must be exactly at the focus of rays; 

 for nearer than this the rays have not yet come together 

 to a focal point ; farther than this they have already 

 crossed and spread out again. Observe (3) that the size 

 of the image will be to that of the object in the exact 

 proportion to their relative distances from the nodal point. 

 (4) Again, as the object comes nearer the lens the image 

 will be thrown farther back, while if the object recedes 

 from the lens the image will approach the lens. (5) It 

 is not every lens that will make a perfect image. It 

 must have a proper shape, and, moreover, it is found that 

 a system of several lenses is better than a single lens. 



Application of these Principles to the Eye. — 

 Now the eye is an instrument consisting of a system of 

 lenses. The eye therefore forms its images of all ob- 

 jects presented to it. In Fig. 74 rays from the two 

 points A and B of an object A B are brought to focus on 



