of the Eye-pieces of Telescopes. 23 



dicular to the paper plane, and the other in the paper plane, and 

 perpendicular to the axis of the lens. 



Now, to estimate the influence of this kind of convergence on 

 vision with a telescope, let Fig. 5 represent the course of the rays 

 in an astronomical telescope, with the common negative eye-piece, 

 adapted to distinct vision of the center of the field of view, for 

 eyes requiring parallel rays. The rays coming from a pointy of 

 an object in the axis of the telescoiie i>roduced, are refracted by 

 the object-glass (which we suppose perfect), received by the first 

 eye-glass, and made to converge accurately to the point F; they 

 then diverge on the second eye-glass, and emerge parallel, and 

 form a distinct image on the retina. But the rays coming from 

 another part of the object pass through some other part of the 

 lens, and are refracted so as to pass through two lines; one hk 

 in the paper plane, which is not generally at the same distance 

 from that lens as the jjoint F; and another perpendicular to the 

 paper plane at g. A similar effect takes place at the next lens : 

 so that the rays of this pencil after emergence are not parallel, 

 but converge to two lines, the convergence of the rays in the 

 paper plane being in most cases more rapid than that of the 

 rays in the perpendicular plane ; or, the line perpendicular to 

 the paper plane being generally nearer than that in the paper 

 plane. When the eye, therefore, is turned to receive these rays, 

 they do not converge to a point on the retina, but converge to 

 two lines within the eye, and then diverge upon the retina. 



Now, if it were in our power to bring the retina nearer to the 

 crystalline (the effects of which may be exactly imitated by pushing 

 in the eye-piece), so as to form sections of the pencil at different 

 points, supposing the ijencil to be circular (as is usually the case), 

 we should have a series of sections similar to those given in Fig. 6. 

 The two lines correspond to the positions of the retina when it is 

 brought ujj to the lines of convergence ; if the pencil be exactly 



