THE REFRACTING TELESCOPE. 



187 



reach us of every possible length between the limits already referred 

 to, and vibrating in every possible plane, so that, even if our lens 

 would make the wave-fronts emerge spherical, it would be found that 

 the long red waves would come to a focus considerably farther out 

 than would the short violet waves, and confusion of the image and 

 colored fringes would result. It is also found impossible to construct 

 lenses with surfaces of any other shape than spherical ; consequently 

 the optician has quite a complicated problem to solve before he can 

 construct an object-glass which will not only make the wave-fronts 

 emerge strictly spherical, but which will also make the red, green, and 

 violet waves unite at the same focus, and thus cause all the waves 

 from each luminous point like a star, which is a sun, like ours, too dis- 

 tant to have visible dimensions, to agitate but a single rod of the 

 retina. 



In practice, this is almost perfectly accomplished by combining a 

 convex lens of crown-glass (the optical name for plate-glass) with a 

 concave lens of flint-glass (the kind used for the finest cut-glass for 

 table-ware), placed close together ; but, to arrive at this result when 

 the lenses are of large aperture, requires an amount of skill and pa- 

 tience attained by few. 



Diagram 9 shows the two most approved forms of object-glasses. 



Diagram 9. 



The first is that used by Alvan Clark, in the largest and most perfect 

 telescopes ever constructed. It consists of a double-convex lens of 

 crown-glass, combined with a plano-concave lens of flint-glass, the 

 crown-glass lens being placed in front. Both surfaces of the crown- 

 glass lens and the first surface of the flint-glass lens have the same 

 curvature. The focal length of this object-glass is nearly equal to 



