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XXIX. Furl'tculnrs respecting the Pnncrutic Eye-Tiile in- 

 vented by Wir.LiAM KncHKNER, M.D., Julhnr of " Prac- 

 tical Ol'servalions on Telescopes, Spectacles," &c. &c. made 

 only by Dollond, London. 



It has long been known, that the magnifying powers of te- 

 K^■=copes could be augmented by increasing the distance between 

 the two glasses next to the eye, and the two that are next to 

 the object-glass, to ulmost double the power of the eye-tube in 

 its u'sual form, i. e. from 30 to 55. This is the utmost that op- 

 ticians have at any time accomplished; yet this variation is so 

 desirable, that I tliink it only requires to be generally known, to 

 be generally desired. 



'* A few months ago, I saw an eye-tube, made by Mr. Cau- 

 choix, with a scale of magnifying powers from 25 to 73; but, upon 

 trial, I found the vision was good only between 35 and 45. 



" My attention was strongly excited by the idea of one eye- 

 tube effecting the whole business of magnifying; and after se- 

 veral experiments, I combined lenses of such proportions that 

 they admitted of being separated from each other so as to mag- 

 nify at one extremity, more than double they did at the other, 

 the vision continuing uniformly distinct to the extreme edges of 

 the field of view. 



''' Having now done more than had been previously effected, 

 I brought it to you. Thcgreat approbation you expressed of what 

 I had done, so encouraged me, that 1 applied unceasingly, de- 

 termined to perfect the object in viewj which I have now ac- 

 complished. 



" I beg to present to you the following accurate measurement 

 of the powers, and faithful account of the performance of 



" The PA^XKATIC Eye-Tube, which gives a neater and bet- 

 ter defined image of a fixed star, and shows double stars decidedly 

 more distinct and perfectly separated, than any other eye-tube, 

 and will enable the observer to determine the distances of these 

 very delicate objects from each other in a more perfect manner 

 than has been possible heretofore. 



" This Eye-tube applied to the Achromatic Refractor of forty- 

 four inches focus, produces, in the most perfect manner, every 

 intermediate degree of magnifving power between 100 and 400, 

 the vision continuing nnifornily distinct to the extreme margin 

 of the field of the telescope. 



" The tube is graduated ; and the change from one power to 

 another may be made instantaneously, even in the dark, with 

 the utmost facility and certainty (sec PI. II. fig. 1 1.). 



*' It is presumed, thai the advantage of my Pancratic Eye-tube 



over 



