528 THE GALILEAN TELESCOPE. 



The formation of the image ^ b l is, however, pre- 

 vented by the interposition of a concave eye-piece eg, 

 which has a small focal length and causes a divergence 

 of the rays from their original direction. The real, 

 inverted image ^ bi is converted by the eye-piece into 

 an erect virtual image a 2 5 2 , which to the eye at o appears 

 considerably larger than the object a b. The formation 

 of the image a 2 b 2 will be understood if we consider 

 in the first instance that rays from a are foiling upon 

 the entire surface of the lens c d, and that all of them 

 are so refracted that they produce an image of a at a lt 

 Among these rays there will obviously be one which 

 passes through the centre of the eye-piece, and whose 

 direction is therefore not changed. Let, in our figure, 

 ca 2 a 1 bQ that ray. Similarly there will be one ray, 

 k m in the figure, which reaches the eye-piece parallel 

 to the axis. But we know that rays parallel to the 

 axis of a concave lens proceed after refraction as if they 

 originated in the focus ; and since / 2 is the focus of the 

 eye-piece eg, the ray km will not proceed towards a^ 

 but towards /, as if really coining from/ 2 . It follows 

 that the rays c a t and k m, which, in the absence of the 

 eye-piece, would produce the image a^ are diverging 

 after having passed the concave eye-piece in the direc- 

 tions towards a x and /, and the eye at o receives the 

 same impression as if these rays proceeded from a 2 , that 

 is, the eye sees at a. 2 a virtual image of a. The rays h ^ 

 and idi are of course also refracted as if they proceeded 

 from #2, but their directions after passing the eye-piece 

 are not represented in the figure, for the sake of clearness. 

 The lenses of telescopes are usually fixed in tubes, 

 which are blackened inside; this ensures steadiness 



