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XII. On the Principle of Construction and general Application of the Negative Achro- 

 matic Lens to Telescopes and Eyepieces of every description. By Peter Barlow, 

 Esq. F.R.S. 8sc. 



Received May 20, — Read May 29, 1834. 



The great advantage which has attended Mr. Dollond's ingenious application of 

 the negative achromatic lens to the micrometer eyepiece, seems to make it desirable 

 that the principles on which that lens is constructed, and its general application, 

 should be more fully illustrated than is done in the short extract made from my letter 

 to Mr. DoLLOND, and given by him in his recent paper in the Philosophical Trans- 

 actions. 



In my original fluid telescope, the negative lens was employed for the double pur- 

 pose of lengthening out the focus and correcting the colour of the front lens ; and the 

 great advantage of the lengthening principle was manifested by the high penetrating 

 power of the instrument in the centre of the field. Unfortunately, however, the per- 

 fect part of this was very limited, so that when Mr. Dollond constructed the second 

 telescope for the Royal Society, I gave up this advantage for the sake of enlarging 

 the field ; but I found that by this means much of the penetrating power of the former 

 telescope was lost ; for although I had the same aperture, many small stars which 

 were before very perspicuous were in this instrument seen only with difliculty and 

 under advantageous circumstances of weather, absence of moonlight, &c. 



This led me to consider whether it would not be possible to retain the advantages 

 I had obtained in the new instrument, and to restore the power of the other principle 

 (that of penetration) by an artificial lengthening of the focus ; but as the rays were now 

 as nearly achromatic as I could make them, it was necessary in this case to have the 

 lengthening lens also achromatic. I had no authority from the Royal Society to make 

 any collateral experiment, but having mentioned my idea to Mr. Dollond, he very 

 readily undertook to construct the small lens, and it was accordingly made and tried ; 

 but owing, as I now imagine, to the imperfect means I had of fixing it, its advantages 

 were not perceived. It, was laid aside, was not referred to in my paper, and would 

 most likely have been altogether lost sight of, had it not occurred again to Mr. Dol- 

 lond to try its effect on the micrometer eyepiece for the Rev. Mr. Dawes. It is there- 

 fore to Mr. Dollond we are indebted for snatching this lens from the oblivion into 

 which I had allowed it to fall. 



It must not, however, be understood that it is only applicable to tliis eyepiece, 

 for it may be applied to any eyepiece, positive or negative, or to the erecting eye- 



