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in the same manner as other Eye-tubes, and is adjusted to distinct Vision by the same Pinion motion.
For the Lowest Magnifying power, the Three Inner tubes must be shut up within the Outer one; when the Magnifying power is to be increased, the smallest of the sliding tubes, A, must be drawn out to either of the numbers engraved upon it; care being taken not to draw out any part of the other sliding tubes, B and C, until the whole of the First, A, is pulled out; - the Second tube, B, may then be drawn out to either of the numbers. engraven thereon; and in like manner the Third tube.
The numbers engraved on the Tubes, denote the Magnifying power of the Telescope.
To change the Power for any less power than the one to which the tubes have been drawn out, the reverse of the above-described mode of proceeding must be observed; - the Largest tube must be returned first, and so on, until they have been brought back to the number required.
Each alteration of the Magnifying power will require a new adjustment of the Pinion; -as the Magnifying power is increased, the dis
tance between the Eye-glass and the Objectglass must be diminished.
"It has long been known, that the Magnifying powers of Telescopes 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 almost double the power of the Eye-tube in its usual form, i. e. from 30 to 55. This is the utmost that Opticians have heretofore accomplished; yet this variation is so desirable, that I think it only requires to be generally known, to be generally desired, both for Convenience and Cheapness*.
"A few months ago, I saw an Eye-tube, made by MR. CAUCHOIX, with a scale of magnifying powers from 25 to 73; but, upon trial, I found that the vision was perfectly good only between 35 and 45.
My attention was strongly excited by the idea of ONE Eye-tube effecting the whole busi
* Before Mr. JESSE RAMSDEN invented, about 1785, The Pipe-drawer for the Terrestrial Eye-tube- and changed the Magnifying power, by changing the two Glasses next to the Eye - for which half Eye-tube he charged 10s. 6d. for every change of Magnifying power, there was the incumbrance and expense of another Eyetube, costing 11. 1s.
ness of Magnifying; and after several experiments, with the assistance of Mr. S. PIERCE, I combined lenses of such proportions that they admitted of being separated from each other so as to Magnify at one extremity, more than double what they did at the other, the vision continuing uniformly distinct.
"Having now done more than had been previously effected, I brought it to You. The approbation You expressed of what I had done, so encouraged me, that I applied unceasingly, determined to perfect the object in view, which I have now accomplished.
"I beg to present to you the following accurate measurement of the powers, and faithful account of the performance of
"The PANCRATIC EYE-TUBE, which I think gives a better defined image of a fixed Star, and shews Double Stars decidedly more distinct*, and perfectly separated, than any other Eye-tube, and I hope will enable us to determine the distances of these objects
* Especially in Achromatic Telescopes, which are, what is termed, a little over-corrected, and the purple rays predominate: that is, when the focal length of the Conver Lens, is formed rather too long for the Concave.
from each other, in a more perfect manner than has been possible heretofore.*
"This Eye-tube, when accurately made, applied to an Achromatic of 44 inches focus, produces, in the most perfect manner, every intermediate degree of Magnifying power between 100 and 400, either for Celestial or Terrestrial uses the Field of Vision continuing uniformly distinct.
"Therefore it is presumed, that the advantage of my PANCRATIC Eye-tube over the usual common Eye-tube, in variety of Magnifying power, convenience, cheapness,and portability, is as 300 to 1.- The cost of a common Eye-tube is One Pound; of the Pancratic only Two Pounds, Two Shillings. "The Tubes are graduated; every 10 degrees, thus, 100, 110, 120, up to 400.
* This may, perhaps, be accounted for, from the greater degree of the aberration arising from the extreme sphericity of the lenses in other Eye-pieces which magnify so highly. It may be said that Common Eye-tubes have rather a larger field but of what use is that part of the field in which objects appear distorted and fringed with Colour. That, can only be considered the actual and useful field of view, the Margin of which, is as perfectly distinct as the middle of the field, when the Telescope is adjusted at an object seen in the middle of the field.