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XI. An Account of a Concave Achromatic Glass Lens, as adapted to the Wired Micro- 

 meter when applied to a Telescope, which has the property of increasing the mag- 

 nifying power of the Telescope without increasing the diameter of the Micrometer 

 Wires. By George Dollond^ F.R.S. S^c. 



Received February 19, — Read February 27, 1834. 



TV^HEN the application of any optical or other arrangement is found to be useful, a 

 correct statement of the manner in which it became so is essentially requisite, in 

 order that each person who may have had a share in bringing it forward may have 

 his due proportion of the merit. 



The achromatic lens which I have applied to the wired micrometer, and which has 

 been found to produce such very considerable advantages to that instrument, arose 

 out of a trial that was made at the suggestion of Professor Barlow, for the purpose 

 of improving the chromatic aberrations which affected the field of the eye-glasses ap- 

 plied to the telescope invented by that gentleman with a fluid correcting lens, and 

 made by myself for the Royal Society. 



The lens in question not being found so effective for his purpose as he expected, 

 was laid aside. It has now been introduced for my purpose, and is made, with some 

 trifling variations, in accordance with his calculations. 



The interposition of a concave lens between the object-glass and the eye-glass of a 

 telescope has been generally known by opticians to produce an increase of the mag- 

 nifying power, in proportion to its focal length and distance from the object-glass : 

 also that a convex lens, if so applied, would diminish the power. 



Except in the Huygenian eye-tube, I am not aware that either of these lenses have 

 been so applied generally, it having been considered that their introduction would 

 materially diminish the light proceeding from the object-glass of the telescope, and 

 also, by deranging the aberrations, disturb the image. 



In the lens I am now describing, these errors are very materially obviated, owing 

 to its being constructed upon achromatic principles*, by which the magnifying power 

 of the telescope is increased in a twofold ratio, without so much diminution of light 

 as is produced by the introduction of a simple lens. 



For example, if the eye-glasses in the original arrangement of the telescope gave 

 100 of magnifying power, the same eye-glasses with the new lens, if I may so term it, 

 will give 200, and the light will be fully equal to that power if obtained by the usual 

 means. The field of view will also be considerably flattened. 



* The discovery of John Dollond, F.R.S., in the year 1758. 



