of tli£ Eye-pieces of Telescopes. 61 



the distinctness will not be much injured, and the distortion will be some- 

 what diminished. 



For a double eye-glass, the two lenses in contact. 



1. For distinctness, the lenses should be two similar crossed lenses, 

 the radii of their surfaces as 1:6, and the more convex side of each 

 turned towards the other. 



2d. The distortion will be diminished by making the first lens 

 (reckoning from the object-glass) equi-convex, and the second meniscus. 



For a positive eye-piece. 



1. Let the two lenses be plano-convex of focal length 3, placed at 

 the distance 2, with the convex side of each turned towards the other. 



2. This distance is somewhat less than that adopted by some of the 

 most eminent opticians. I prefer it because there is a smaller chance 

 of seeing the dust, &c. on the field-glass, which, when the lenses are 

 nearer, happens frequently, especially if the observer be near-sighted. 



3. The distance of the field-bar from the first lens must be f. 



For a negative eye-piece. 



1. For distinctness, let the first lens be a meniscus lens of focal 

 length 3, the radii of its surfaces as 11 : 4, and its convexity towards 

 the object-glass. Let the second lens be a crossed lens of focal length 

 1 ; the radii of its surfaces as 1 : 6, and its more convex side toward 

 the first. 



Let the distance of the lenses be 2. 



2. If plano-convex lenses be used, there is a considerable loss of 

 distinctness, and no diminution of distortion. 



3. The field-bar should be at the distance 1 from the eye-glass. 



i. Any other proportion of the focal lengths of the lenses may be 



