THE REFRACTING TELESCOPE. 



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ency of light to move in straight rays. After the emerging waves 

 have run to a focus, they diverge again from this focus as a new cen- 

 ter, with spherical fronts, and in exactly the opposite direction to 

 that from which they arrived, just as if the light emerging from all 

 parts of the lens was propagated through and heyond the focus in 

 straight lines ; hence the marginal portion of the converging and 

 diverging wave-fronts on each side of the focus will form two cones, 

 turned in diametrically opposite directions, their common apex being 

 the common center of the spherical wave-fronts, viz., the focus of the 

 lens. 



It is now evidently a simple matter to place a second lens at such 

 a distance behind the focus of the first lens that it will transform the 

 spherical wave-fronts diverging from this focus into plane wave-fronts, 

 parallel to those entering the first lens ; and, because these waves 

 emerging from the second lens have plane fronts, they must, if they 

 are allowed to enter the eye, come to a focus on the retina, and cause 

 the eye to see a point of light, for precisely the same reason that it 

 would see that point if the two lenses were removed, and the direct 

 light from the vibrating molecule were allowed to enter it. 



This is the principle of the refracting telescope ; the first lens rep- 

 resents the object-glass, and the second lens the eye-piece. 



The Diagram 6 represents the object-glass, the eye-piece, and the 

 eye, in their proper relative positions ; also the light-waves from an in- 



Diagram 6. 



finitely distant vibrating molecule entering the object-glass, emerging 

 from it with spherical wave-fronts, which converge to a point of 

 great agitation or focus, whence they diverge with spherical fronts, 

 until, by passing through the eye-piece, they are converted into plane 

 wave-fronts ; thence, entering the eye, they come to a focus on the 

 retina. 



The diameter of the pupil of the eye being one fifth of an inch, the 

 eye-piece must be of such a focal length that it can be placed so near 

 the focus of the object-glass that the diameter of the emerging cylin- 

 der of plane wave-fronts shall not exceed one fifth of an inch, else the 

 cylinder of light entering the object-glass will not be reduced in 



