i of the Eye-pieces of Telescopes. ;j 



image are formed at different distances from the eye : in this case, 

 no distinct image is formed at all, except in the center of the field ; 

 the rays of other pencils never converging accurately to a point. 

 This defect may frequently be entirely corrected, though it is some- 

 times prudent to leave it partially uncorrected. 



But upon pursuing the investigations, it will appear that these 

 conditions frequently interfere with one another, so that the con- 

 struction, which is most advantageous for obviating one defect, 

 makes another pretty large. And this has occasioned some 

 anomalies in the practical rules which workmen have established. 

 The lenses commonly in use are double equi-convex lenses, and 

 plano-convex lenses : and the general rule of workmen is to place 

 these in such a manner that the angle made by the incident ray 

 with the first surface, shall be nearly equal to the angle made by 

 the emei'gent ray with the second surface. This I believe is to 

 diminish the distortion ; and, for that purpose, the rule is not very 

 far from the truth. But there is a remarkable departure from this 

 law in the constru«tion of the common eye-piece (Ramsden's) for 

 transit instruments, and, generally, for all telescojies to which 

 micrometers are applied. The eye-glass, nearest the object-glass, 

 is a plano-convex, with its plane side towards the object-glass. 

 This construction (which makes the angles above-mentioned ex- 

 tremely unequal) is adopted for the purpose, as workmen express 

 it, of procuring a flat image ; that is, for the purpose of making- 

 all parts of the image, after refraction, at the same distance from 

 the eye. In this construction, then, the first condition is given 

 up for the second : and the third is not at all considered, no general 

 rule having been given for it. 



It is necessary, then, to consider on which of these conditions 

 the greatest stress should be laid. When the aperture of the teles- 

 cope is very small, or its magnifying power very great, the breadth 

 of a pencil is very small, and the second and third defects become 



A 2 



