206 MR. BARLOW ON THE NEGATIVE ACHROMATIC LENS. 



piece, or indeed to any telescope of fluid or glass, or to refractors ; for it is, in fact, 

 not a part of the eyepiece, but of the telescope itself : and it is for this reason its 

 advantages are so conspicuous in the application Mr. Dollond has so ingeniously 

 made of it ; for by lengthening the focus before the rays arrive at the eyepiece, the 

 image is magnified, while the wires retain only their original size. 



Having thus shown the origin of the negative achromatic lens, I may be allowed 

 to state the motives and reasonings which guided me in the computation of the 

 curves, and what appears to me to constitute the advantages it is found to possess. 

 Notwithstanding the extreme difficulty there is in constructing an achromatic object- 

 glass, yet with perfect materials the difficulty is only in the manipulation ; and this 

 being overcome, there is not so great a natural impediment to perfection in this part 

 as in the eyepiece, — for we know that it is impossible to make a perfect positive 

 power * ; and if the same absolute impediment does not occur in the negative eye- 

 piece, yet the thicknesses of the lenses render the task very difficult, not only to exe- 

 cute, but to compute the proper curvatures to ensure perfection. If this view of the 

 case be correct, we see at once the advantage of magnifying the object as much as 

 possible before we apply the eyepiece ; and this, in fact, is the whole theory of the 

 negative achromatic lens : that is, supposing the rays to be rendered achromatic by 

 the object-glass, they are intercepted by the negative lens before they cross, which, 

 being itself also achromatic, extends them to any length, and thereby produces the 

 effijct of lengthening the whole focus in the same proportion, and consequently the 

 power of the telescope, the eyepiece remaining unaltered. 



In the conclusion of my letter to Mr. Dollond, I have offered a suggestion, whether 

 it would not be possible to retain the same eyepiece for all powers by changing only 

 the negative lens. This must of course, as he has observed, change the scale of the 

 micrometer ; but this being changed, by so adapting the lens as to render the powers 

 simple multiples of each other, would not, I conceive, be attended with any disadvan- 

 tage. In other cases, where a micrometer is not employed, and where the utmost 

 perfection is not looked for, every variety of power may be produced by simply moving 

 the negative lens nearer to or further from the eyepiece ; for both the object-glass 

 and lengthening lens being achromatic, the image, wherever the focus is formed, will 

 be achromatic also ; and the spherical aberration of the lens is so inconsiderable, as 

 only to be discovered by the most perfect eye, when removed from that point in which 

 it is computed to be perfectly corrected. The negative lens is therefore admirably 

 suited for day telescopes with correcting eyepieces, as also for astronomical tele- 

 scopes where the micrometer is not applied ; for by giving an adjustment to the 

 lengthening lens, the power may be changed in any proportion, even without removing 

 the eye or losing sight of the object. I have no doubt that these and other applications 

 of the lengthening lens will be made, and amongst others, I am willing to hope that 



* See Professor Airy on the Eyepieces of Telescopes, Transactions of the Cambridge Philosophical Society, 

 vol. iii. 



