1882.] of a negative eye-piece. 'loo 



chroraatise the rays, if I may use such a phrase, iu order that they 

 may be achromatic when they have passed through the eye-glass. 

 The place of its focus however, as far as I can see, ought not to be 

 changed : it is one easily ascertained : it is the focus for rays not 

 parallel, but with the convergence given them by the object-glass 

 at the point where the lens intercepts them to the greatest ad- 

 vantage. Knowing the focal length of the object-glass and of the 

 field-glass, it seems to me that this point ought to be carefully 

 ascertained by the maker, and indicated by the stop : my French 

 sextant-telescope has wires at this point which I can get perfectly 

 distinct by adjusting the eye-glass; and I think without distortion, 

 though theoretically it seems this ought to be a result, Any how 

 the eye-glass should be made moveable as I thought myself, or 

 else should be altered in form as was suggested to me by a friend, 

 i.e. made less convex for a short-sighted person, more so for one with 

 long-sight ; so that the person using the telescope, by first focussing 

 on the stop, will have as perfect vision as possible. When I men- 

 tioned the difficulty, to a friend or to my maker, though I do not 

 remember which, the remedy he suggested was, that I should 

 focus on the stop: and this is no doubt correct, but I am not sure 

 that this prescription is sufficient of itself. Mr Coddington says 

 that the stop is to be placed half-way between the two lenses, the 

 field-glass and the eye-glass: but this is not actually done in prac- 

 tice ; it being placed much nearer to the eye-glass than to the 

 other : and also that the two lenses should be memscws-shaped, 

 the field-glass with the surfaces of radii 4 and 11, the convex side 

 turned towards the object-glass : and the eye-glass with surfaces 

 6' and 1, making what is called a crossed lens, the convex side here 

 also being turned towards the other glass : and both these princi- 

 ples, I may add, seem borrowed from the rules given in Sir Geo. 

 Airy's paper on "Achromatic Eye-pieces" in Vol. III. of our Transac- 

 tions, read in 1827. To my mind, the stop ought to be fixed by 

 the relative focal-lengths of the object-glass and of the field-glass: 

 and the eye-glass may vary in distance from it according to the 

 nature of the observer's eye. 



The following objection has been taken to making the eye- 

 glass moveable : that the achromatism is injured by doing so : but 

 it seems to me that the great thing is definition, and that the eye 

 itself may possibly rectify a slight defect in this respect. At the 

 same time an alteration of the form of the eye-glass to suit a short- 

 sighted eye seems a most proper thing to introduce : the focussing 

 on the stop being reserved to determine finally its position. A 

 positive eye-piece is, I believe, not achromatic. 



To sum up : my principle is this. The field-glass, of a defined 

 focal length, placed in front of an object-glass of defined focal 

 length, produces the best defined image at a definite point. The 



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